by Rex Parker, PhD director@princetonastronomy.org
October 13, 2026 Meeting at Sherrerd Hall. The nor’easter storm here in September and multiple hurricanes in the Pacific remind us of the power of the sun coupling with earth’s atmosphere to create large storms persisting for days and weeks. Now as autumn’s golden glow and earlier sunsets work their wonder on our psyches, and hopefully yield to clearer night skies, it’s time to roll up our sleeves and get back more deeply into astronomy. Let’s aim for a strong turnout in person for the October 13 meeting of our club. We aspire to be more than another virtual presence offering popular astronomy on the internet. Our club has met in person continuously on a monthly basis for over 60 years (except those few times when we winter storms did prevail). It is up to you, our members, to help continue that tradition by participating in person whenever possible. Of course it isn’t always possible, and this meeting will be stream by Zoom and You Tube from the live event. For more on the guest speaker for Oct 13, please see Victor’s section below. After the main talk we will continue our discussions into current topics in astronomy and club and observatory activities.
Venue and parking for the Oct 13 meeting. Our long-time venue Peyton Hall (home to the Princeton Astrophysics Dept) remains under reconstruction, and we hope to return soon. Until then, thanks go to Bob Vanderbei and Princeton’s Dept of Operations Research and Financial Engineering for the privilege of holding our meeting at Sherrerd Hall (picture below). Parking lots and streetside spots are very close on William Street, and Sherrerd is less than a block east of the street, #4 on the map here: https://www.princetonianamuseum.org/artifact/7102bbbd-5b04-4138-9c2b-5cd9b425da73. I hope to see you there on Oct 13.
The architecture of Sherrerd Hall is unique, making it easy to find just east of William St.
Hot Topics for Our Meetings. You are probably the go-to person in your family and circle of friends when it comes to emerging astronomy themes. The popular press and media have been providing abundant PR for supermoons, planet alignments, and meteor showers, but they don’t usually go into the physics. We need your participation each month to help identify in advance the upcoming astro topics soon to emerge in the popular press and media. Please send your thoughts and themes for hot topics by email to me at director@princeonastronomy.org.
Dark Matter Illuminated, part 2. As a primer for November’s guest speaker, I would like to go deeper into a different theory addressing the galaxy rotation problem in astrophysics which appears to fit at least some of the data better than the cold dark matter theory (also called lambda CDM). This is called Modified Newtonian Dynamics (MOND). In the Jan 2026 issue of Sidereal Times (link: https://princetonastronomy.com/2026/01/03/from-the-director-156/) I described how the dark matter theory evolved and mentioned MOND as an important alternative theory in cosmology. Recently MOND has been getting more attention in the astrophysics community, perhaps in part because of the challenges of identifying what dark matter could actually be. Most dark matter candidates are a type of particle not yet known to physics. Several experiments are underway searching for the fundamental particle comprising dark matter, such as the LUX–ZEPLIN (LZ) experiment I described at the September AAAP meeting, an ultra‑sensitive dark matter detector searching for weakly interacting massive particles (WIMPs) using 7 tonnes of liquid xenon in a gold mine nearly a mile deep in South Dakota. But so far these amazing physics experiments have not yielded a positive result. Here I’d like to show in a simplified way how MOND could better account for at least some of the key data that led to the dark matter theory in the first place.
The Israeli physicist Mordehai Milgrom, who worked at the Weizmann Institute and also at the Institute for Advanced Study here in Princeton, had a different take on the enigma which arises from galaxy rotation speeds. In spiral galaxies the speed of stars and gases is controlled by gravity, but the velocities out in the perimeter do not drop with radius as expected from Newtonian gravitational physics. Rather than adding an invisible ingredient, dark matter, Milgrom proposed that Newton’s law of gravity itself might behave differently at a galaxy’s edge. The key is that out in these regions, the gravitational acceleration of stars and gas is far lower than in the crowded inner parts of a galaxy, and far lower than anything in our neighborhood of planets. You’d have to travel about 7,000 times Earth’s distance from the Sun, well into the Oort cloud, before the Sun’s gravity fades to the MOND level.
Just how faint is that? Physicists put the threshold at about 1.2 × 10⁻¹⁰ meters per second squared, roughly 80 billion times weaker than the 9.8 we feel on Earth. A ball dropped on Earth falls a meter in under half a second. At the MOND threshold, that same ball would need a day and a half to drop a meter. Since we can’t reproduce these conditions here or reach them with planetary probes, MOND theory is tested on galaxies and on wide binary stars, where the results are still hotly debated.
Put more simply, dark matter keeps the laws of gravity intact and adjusts the mass, while MOND keeps the mass as observed and adjusts the law. Since Newton’s law is a foundation of physics, that makes MOND at least as revolutionary as invisible mass. Its strength is that from visible matter alone, it accurately predicts spiral galaxy rotation speeds with no invisible halo needed for each galaxy. MOND has its own puzzles, though, and they are being studied intensely. Two areas in astrophysics where dark matter seems to fit better than MOND are the gravity-driven movements in very large-scale clusters of galaxies, and in patterns found in the cosmic microwave background (see the January article cited above). On November 10 our guest speaker, Dr Stacy McGaugh of Case Western Reserve University, will show us how the astrophysics world sees MOND and where it stands along with dark matter theory in these profound areas of cosmology.
Astrophotography Beyond the Fundamentals The October, 2026 monthly meeting of the Amateur Astronomers Association of Princeton will take place in the lecture room in Sherrerd Hall on the campus of Princeton University on Tuesday, October 13th at 7:30 PM. As usual, the meeting is open to AAAP members and the public. Participants can join the meeting in-person at Sherrerd Hall or log in to the Zoom session as early as 7:00 pm to chat informally before the meeting begins. The evening’s guest speaker will be astrophotography expert and author Charles Bracken who will help to expand our knowledge of astroimaging beyond the fundamentals we learned at last month’s meeting.
Options for Attending the Meeting You may choose to attend the meeting in person or participate via Zoom or YouTube as we’ve been doing for the past few years. (See How to Participate below for details). Due to security concerns, if you log in before the host has set up internet connectivity on-site, you may need to wait in the Waiting Room for a few minutes until the host is prepared to admit you into the meeting. You’ll need to unmute yourself to make comments or ask questions. It’s polite, though not required, for you to enable your camera so other participants can see you. The meeting will be recorded and edited for posting to our club’s YouTube channel.
Join us for our “meet the speaker” dinner Mr. Bracken will join club members at Winberie’s Bar and Restaurant, 1 Palmer Square, for our traditional “meet the speaker” dinner. Our reservation is for 5:45 pm Tuesday, October 13. Please contact Tom Angle (tcangle@icloud.com) to let him know you plan to attend.
Here’s the anticipated agenda for October 13, 2026’s monthly meeting of the AAAP:
(Times are approximate)
Capturing Every Object Worth Imaging Featured Speaker:
What astronomical objects are good targets for astrophotography? Which are most fulfilling to capture in images? Will you ever run out of objects to image? Which objects are most overlooked by astro-imagers and why? Charlie Bracken describes his quest to research, capture, and process images of more than 1,700 deep-sky objects for his book The Astrophotographer’s Universe. He’ll describe lessons learned and his perspectives on this technical and aesthetic passion.
Charles Bracken Charlie is the author of several books on astrophotography, including The Deep-sky Imaging Primer, The Astrophotography Sky Atlas, The Astrophotography Planner, The Visible Universe, and The Astrophotographer’s Universe. He has been imaging for more than 15 years, and he has lectured on astrophotography globally, helping people get the best out of their equipment and skies. He lives in Bucks County, PA and also images via remote setups in Utah and Chile. You can find him on Astrobin (CharlesBracken) and his website is digitalstars.wordpress.com.
Mr. Bracken will have copies of “The Astrophotographer’s Universe” for sale and signing at the meeting.
How to Participate (Links) Zoom& YouTube Live Topic: October 13, 2026 AAAP Club Meeting-Charles Bracken Astrophotographer Time: Oct 13, 2026 07:00 PM Eastern Time (US and Canada) Join Zoom Meeting Meeting Chat Link Meeting ID: 839 4409 5122 Passcode: 005635 Join Instructions
AAAP Video Library AAAP’s library of monthly meetings is available on the club’s YouTube channel. Last month’s meeting featured Dr. Michael DiMario and Prof. Robert Vanderbei presenting “Fundamentals of Astroimaging.” The edited video (AAAP Sept 8 2026) is at http://youtu.be/UfTktZlx5Ao and runs 1:48:17.
A look ahead at future guest speakers:
Date
Featured Speaker
Topic
October 13, 2026
Charles Bracken Astro-photographer and author deepskyprimer@gmail.com
Mr. Bracken is an amateur astrophotographer and author of “The Deep Sky Imaging Primer” now in its third edition. He will present his perspectives on astroimaging.
We expect to have copies of his book(s) for sale for the author to sign at the conclusion of his presentation.
Prof. McGaugh seeks alternatives to cold dark matter to explain where the universe’s “missing mass” may be secreted. He’ll be joining us virtually.
Thanks to Rex Parker for suggesting this speaker.
December 8 2026
Joel Hurowitz Associate Professor Graduate Program Director Acting Principal Investigator, Mars 2020 PIXL Team Department of Geosciences Stony Brook University joel.hurowitz.stonybrook.edu
Prof. Hurowitz will speak virtually to our club about his research using the Perseverance rover and about astrobiology on Mars.
Prof. Phillipson, originally scheduled to be October 2025’s guest speaker, is an unconfirmed prospect to try again in 2027
Prof. Phillipson leverages statistics, nonlinear dynamics, and machine learning to study the explosive and highly variable characteristics of exotic astrophysical objects such as black holes and neutron stars.
Thanks to Bill Thomas for suggesting this speaker.
As always, members’ comments and suggestions are gratefully accepted and much appreciated. Thanks to Ira Polans and Dave Skitt for setting up the online links and connecting the meeting to the world outside Peyton Hall.
Hello AAAP Shoppers. We hope you have been enjoying your AAAP logo apparel and items. We will be shifting the catalog to include more winter items the week of 10/19. If you are planning to purchase an item, just remember that the items take about 2-3 weeks after order to be delivered. If you need assistance with a custom size or color, please email me at rjsherman@hotmail.com.
How are the black holes created? How big or small are they? What is an “event horizon” after all? How many black holes are there? Will our very own Sun end its life as a black hole? Is there a black hole at the center of milky way that we all revolve around (the answer is yes)? Did the Big Bang originate from a black hole? And will it all ultimately come to an end as a black hole?
This handy-dandy book covers a lot of ground not only about unravelling the mysteries of black holes, but astronomy and astrophysics in general as well. The author, an astrophysicist at Oxford University, has a witty writing style that livens up abstract topics that can otherwise be rather dense to comprehend. At about 260 pages, the book also covers a lot of ground for the history of astronomy and how it’s been like peeling the layers of an onion especially over the past 500 years or so since the times of Galileo. Remember the demotion of Pluto as a dwarf planet? There’s a whole section devoted to how that came about and stirred up a maelstrom of emotions around the world (and still does).
As a closet astrophysicist and a retiree with ample time to ponder over the mysteries of the wide universe, this books talks to me.
The popular sci-fi TV series “Star Trek” recently celebrated sixty years. I was probably too young to enjoy “Star Trek”. As a result I did not develop any passion or following for StarTrek. I was however fascinated with the Star Wars movie franchise instead. George Lucas extended his original trilogy of movies to another trilogy before selling off the franchise to Disney.
Lucas is an unconventional movie maker, and a very creative one. Each of the six movies in the franchise were not just brand extensions. Each was creative in its own way. Rich in concepts like the Jedi, and the imagination, featuring a variety of characters like the droids R2D2 and C3PO, and Spacecrafts like Millenium Falcon and Star fighter. Several different planets are involved in the plots. Galactic hopping to get from one location to another using higher level conceptual propulsion systems are very imaginative.
The propulsion systems are the key because of the limitation that no object can move faster than the speed of light. The astronomical distances take several light years to traverse. The speed of light itself is a very high number. Even travelling at those speeds takes years. Galactic travel would take a lot more time. Some imaginative propulsion systems like hyper drive are required to break the speed limit and shrink the travel times. Or taking some short cuts in space via worm holes. That is the biggest challenge of science fiction and of course reality too – how to travel large distances without taking too much time.
The visuals of different planets, different ecosystems, different vehicles moving along. It is simply wonderful. The plot of course is the same old one of good vs. evil. Fights using light sabers, or different space crafts. The way this was accomplished using the technologies of the seventies and eighties is amazing.
I would like a more co-operative plot rather than a contentious one. People working as a team to solve intractable problems amongst imaginative backgrounds. Good vs. evil is a stale plot but no one realizes nor objects to it. Each time we seem to relish it. The movie makers, including Lucas, appeal to human emotions and make money via making the movies. Bones of contention will continue to be part of plots for a long time to come. Unless human emotions change.
Lucas made his movies at Skywalker ranch near San Francisco, while the mainstream movie making business operated out of Hollywood in Southern California. After Lucas sold off his franchise to Disney, it started capitalizing on its recent purchase. Extending the brand as much as possible. While Disney did use some creativity in making new movies in the franchise, it did not capture my imagination. Disney dedicated a part of its theme park Disney Hollywood Studios to Star Wars. In Orlando, FL, this dedication to the original Star Wars series is commendable.
Recently, Lucas opened a new museum of narrative art in Los Angeles. The museum, long in making, moved its location a few times before settling down in Los Angeles. This museum is not all about Star Wars, although Star Wars features in a big way. It is a general museum about narrative art.
I hope to visit Los Angeles sometime to visit the museum. The picture shown below, courtesy Lucas Museum of Narrative Art, shows the curvaceous design of the museum located in Southern Los Angeles, with the downtown of high rise buildings in the background.
Editor’s comment Thank you, William, for sharing this stunning photograph. I have to admit, I’m a little baffled by it—in the best possible way. On one side is the breathtaking beauty of the night sky, with our home galaxy stretching across the darkness; on the other is that powerful beam of human-made light, a reminder of how much artificial light we have added to the night. It also brings to mind Iceland’s Imagine Peace Tower, Yoko Ono’s beautiful but very bright annual tribute to peace, as well as the glow from Iceland’s many greenhouses, where artificial lighting helps grow tomatoes and other crops through the long, dark winters.
Perhaps the photograph is a wonderful reminder of our complicated relationship with light—we create it for art, food, safety and inspiration, but sometimes forget how precious the darkness is, too.
NGC 7331 is an unbarred spiral galaxy located in the constellation Pegasus, approximately 43.79 million light-years away from Earth. It is often referred to as “the Milky Way’s twin” due to its similar size and structure.
The galaxy’s central core is pronounced and contains a supermassive black hole.
This galaxy has recently hosted multiple supernovae, including the notable supernova 2026aaiv discovered on September 1, 2026 shown by the two red lines pointing to the star.
Just a few arcminutes away in the sky, a collection of smaller, faint galaxies are near NGC 7331. Collectively, these galaxy neighbors are nicknamed the “Deer Lick Group.” These galaxies are called the “Deer Lick Group” because NGC 7331 is like a “salt lick” attracting a handful of deer. However, this is only a chance alignment – NGC 7331 lies approximately 44 million light-years from Earth, whereas the four “deer” range from 290–350 million light-years from Earth.
What is very interesting on the September 1, 2026 supernova discovery, it that it was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) automated survey. This is a group of telescopes located in Chile, Hawaii, and South Africa to locate near earth objects or asteroids on their way to Earth to provide an early warning system.
We all like to look at the night sky: just watching the stars and the constellations is enough to spark wonder and curiosity in our minds. But sometimes, you wish that you could have a closer look at objects and see some objects that you cannot see with the naked eye, and one way to see them is through astrophotography. Nearly 4 months ago, I purchased the Seestar S30 Pro Smart Telescope, which is specifically for astrophotography, and it has helped me capture various deep-sky objects (DSOs). Clicking these objects made me wonder about what stories lie behind these objects, and in this article, I will not only share astrophotographs clicked by me, but I will also share the stories behind them.
NGC 7000 (North America Nebula)
Discovered by William Herschel in 1786, this nebula is called the North America Nebula for its resemblance to the continent of North America. The nebula is 90 light-years across and, if you could see it with the naked eye, would appear several times wider than the full moon! It is located 2,600 light-years from Earth and is an emission nebula (and a star-forming region) located in the constellation Cygnus.
Messier 51a (Whirlpool Galaxy) and Messier 51b
The Whirlpool Galaxy, or M51a, is recognizable by its spiral structure and bright center. However, it is not alone. Messier 51b is its companion (the smaller, yellow-looking galaxy to the right) and is interacting with the Whirlpool Galaxy gravitationally. Both the galaxies have been actively colliding and interacting for millions of years, and eventually, the two will merge to become one galaxy.
Messier 8 (Lagoon Nebula) and Messier 20 (The Trifid Nebula)
The Lagoon Nebula (Messier 8) is a famous nebula in the constellation Sagittarius and is famous for its striking pinkish-red colors. Discovered by Giovanni Battista Hodierna and catalogued by Charles Messier, the nebula acts as a stellar birthplace and frequently gives birth to new stars, and the high levels of UV rays from the starbirth cause the gas in the nebula to look pinkish-red. It is called the Lagoon Nebula because the bright star in its center, Herschel 36 (which is 200,000 times brighter than our sun), and the rest of the nebula are separated by dark dust lanes, looking like a lagoon. The Trifid Nebula, on the other hand, was discovered by Charles Messier in 1764 and was named the Trifid Nebula by John Herschel (Trifid means something divided into three parts, and the prominent black dust lanes do divide the nebula into three parts). For the same reason as the Lagoon Nebula, the intense UV rays from starbirth cause the nebula to glow pinkish-red.
As we think of these stories of celestial objects, it is important to think back and reflect upon the fact that we still don’t know too much about our universe, and that is why it is important to be constantly exploring the universe and contributing to the field of space science: after all, the universe has given itself the power to figure itself out, and let’s work towards this task, one step at a time.
I have finally gotten into astrophotography with my SeeStar S30 Pro, which I’ve had for a few months now. I am amazed at the images it has captured so far just on its default settings (alt/az mount and 10-second subs)!
In ancient times, numbers used to be natural numbers also known as counting numbers which start from ‘1’.
The concept of assigning a value or a number to “nothing” did not exist. There used to be numbers for numerical values, but the question was, what if one had nothing. How could one give nothing a value?
The mathematician Brahmagupta lived in the 600’s in Rajasthan, India. He studied the stars and did math. One day he questioned: What if nothing could be a number?
At that time people believed zero just represented blank space.
Brahmagupta saw it differently. He wrote a book called the Brahmasphutasiddhanta, which means “The Opening of the Universe.” In it, he did something interesting. He used zero like a real number.
He talked about how to add, subtract and multiply with zero. Additionally, he made rules on how to divide with zero. This helped create the foundation of the number system we still use to this day!
Later, Brahmgupta’s work spread to the Middle East and then to Europe. Binary Systems and the basis of Computers are very much based on the foundational principles laid down by Brahmgupta in the 7th Century!
“The Great Attractor Is Drawing the Milky Way at 1.3 Million Miles per Hour (580 km/sec) — What Does It Mean for Our Galaxy?
Learn about the Great Attractor, a region of the universe that is constantly pulling the Milky Way and thousands of other galaxies”.
Written by Jack Knudson“
“Right now, a faraway region of the universe is pulling the Milky Way and neighboring galaxies toward it. We’re technically moving at blistering speeds of around 1.3 million miles per hour in its direction, but we can’t feel this movement at all. The mysterious area that’s drawing us in, called the Great Attractor, has intrigued scientists for years.”
“This is where the Great Attractor comes in. In the 1970s, researchers noticed a significant anomaly that had materialized in a dipole pattern: The space ahead of Earth appeared hotter, while the space trailing behind appeared colder, according to the University of Southern California. Scientists have since determined that this is likely not an intrinsic effect of the Big Bang, but an illusion caused by the Solar System speeding through space.
“Regardless, one thing became clear: the Solar System — and our whole galaxy — was being pulled by some unknown force.”
“One way to envision the Great Attractor’s influence is to look at it as a hierarchy. The Local Group of galaxies, which includes the Milky Way, is at the bottom rung, being pulled toward the Virgo cluster. These two are both being pulled toward the larger Virgo Supercluster, which itself is being pulled toward the Laniakea Supercluster.”
“All this pulling comes from a central gravitational point in the Laniakea supercluster, which is attracting the aforementioned galaxy clusters due to its immense mass.
It might be hard to believe, but this isn’t even the end of the hierarchy. Scientists believe that the Laniakea supercluster is pulled by the Shapley super cluster, an even larger collection of galaxy clusters with a center that’s about 650 million light-years from Earth. In short, not even the puller is immune from being pulled.”
“Jun 4, 2026, 6:45 PM| 3 min read”
Comments: Being pulled or pushed is an incorrect description. Everything from inside an atom to large scale structures has inherent motion, which is conserved. Motion follows the gravitational terrain in orbits or freely moves if it exceeds an escape velocity. IT will move across the universal terrain between systems. The frequency shifts do to motion are not illusions.
This opens up the following observations and questions:
Do the spectral shifts of the 100,000 to 300,000+ galaxies in the Area of the Great Attractor compromise Hubbell’s constant? The Hubble constant is based on a statistical correlation analysis which is used to process a set of data to a linear or possibly second order relationship. The start of the model should be random to establish the base mean and STD. No cherry picking. Additional data can be added as it is discovered. It is expected that additional data over time will change the mean. The slope or mean is only about the data not any theory or empirical physics equation result.
Special relativity requires any motion t o be described in the frame of the observer (o). No preferred observations. The description above is based on the attractor drawing Earth (o)? To frame the system correctly, the attractor (e) is moving away from the observer on Earth. If it is moving toward Earth then it should be blue shifted. The stated shifts were derived from using the wave length ratio z. z=(λe-λo)/λ0 = λe/λo-1 → the range of λo is λe→∞. This calculation is very non linear and not proportional? It cannot be mathematically accurate to calculate velocity without converting to frequency (energy). There is no law of the conservation of length. There is a law of the conservation of energy using frequency
The only proper accurate equation is the Relativistic Doppler equation which also corrects the clock differences do to the motion and combines the blue and redshift first order equations in to a single second order equation to describe the total system being studied.
Compare using wave length or z versus frequency:
Z Observed (qualitative) metric of increased wave length as meters as velocity increases:
Range observed isλo→to∞.
Completely non linear and non proportional. Only converts to frequency with error do to∞ in the denominator range. It is a qualitative metric which is the inverse of energy.
Does not matchup with v/c an empirical linear proportional function.
Red shift or z originates from early observations as a rough qualitative comparison of observed objects
Limited, the units are meters which contain one degree of information.
Is not very suitable to calculate an accurate velocity.
6. Not directly compatible with law of conservation of energy. No conservation of wave length.
Frequency
Represents energy as Planks Equation. Completely linear and proportional tovelocity.
Range offo isfe→0=1
Matches up with v/c 0→c =1
More dimensions of information seconds and cycles (c/λ) . Represents energy.
Completely accurate to calculate velocity subject to clock correction at higher v.
Conservation of energy applies. Can be set up as an energy balance since every redshift is balanced with a blue shift on the far side observation.
Relativistic Doppler equation:
Combines the first order red and blueshift equations and corrects the different clock rates.
Is the complete description of this system of red and blue shift with a second order equation that plots in 2 quadrants +v and –v.
Comments:
I have noticed several different Hubble values which were used in the spreadsheet ±.05 %. The Hubble and Web telescopes have different values for objects further away. A correlation analysis of the data is only about the data. There is no law of nature involved. Unless there is proof of the correlation and it is tested, it should not be used for precise calculations. The fact that 65 galaxies in the Virgo Cluster are blue shifted makes a point. This data is not even in the same xy quadrant. The next paper admits various cherry picking, omitting closer observations and for other reasons. If the observer was in the Virgo cluster and they same thing was done etc. You would not have a very good model. If this data is used then the model will have a high STD or variation. The mean would change. The stock market predicted seven of the last three recessions!!! In Industry we use SQC (statistical process control) to model an established process. When a 6 point trend ± compared to the mean happens, an adjustment has to be made to bring the process back to the mean. If a data point is beyond ±3 standard deviations then someone has to figure out what happened. The statistics are only about the data produced. It doesn’t tell you what’s going on
Comments on the excerpts following paper: some letters and words did not transfer.
The density and peculiar velocity fields of nearby galaxies Michael A. Strauss School of Natural Sciences, Institute for Advanced Study, Princeton, New Jersey 08540 and Jeffrey A WillickObservatories of the Carnegie Institute of Washington, 813 Santa Barbara Street, Pasadena, California 91101-1292
“The Hubble Law states that at distances much less than the Hubble radius, the expansion of the universe causes the recession velocity of a galaxy cz to be proportional to its distance r: cz = H0r; (1) where H0 is the Hubble Constant, whose value remains uncertain by a factor of two; in astronomer’s units, it is often written as H0 = 100h km s1 Mpc1 , the quantity h parameterizing our ignorance of its value. Mpc stands for mega parsecs, the common unit of distance for much extragalactic work: 1 Mpc = 3:08 1024 cm. Thus in physicist’s units, H0 = 3:25 1018 h sec, or H1 01010 h1 yr. In practice, we will rarely be troubled by the uncertain value of h in this review, because we will measure distances in units of km s1 , wherein H0 1. The observational evidence for the linearity of Eq. (1) is reviewed in Peebles (1993) and Lauer & Postman (1992); cf. x 3.6 below. At the low redshift discussed in the majority of this review, relativistic effects are for the most part unimportant and the Hubble law is an excellent approximation. However, galaxies have motions above and beyond their Hubble velocities, deviations from the isotropic expansion that holds only in the theoretical idealization of a perfectly homogeneous universe.
Another definition of the local universe is that within which evolutionary effects in the galaxy properties can be assumed to be negligible. In practice, we will restrict ourselves to recession velocities below 20,000 km s1 (z = 0:067)
The Hubble velocity and radius are just stops or fixes put in as the equation goes extreme. It appears generally that further objects are move faster; If Special Relativity was used there is no need for the Hubble velocity and radius. The Hubble Law is just approximate.
2.1 The Big Bang Model and its Parameters It is an observational fact that all galaxies (with the exception of galaxies in the Local Group and a few galaxies associated with the Virgo Cluster) have positive redshifts, and it is observed that redshifts are proportional to distance (x 3.6). This is interpreted as due to the expansion of the Universe. The Cosmological Principle, as formulated originally by Einstein, states that on large enough scales (to be quantified below) the Universe is homogeneous and isotropic; this model together with the tenets of general relativity leads to the prediction that we do not live in a static Universe 1 . In particular, the Cosmological Principle implies that the covariant line element between two points is given by: ds 2 = c 2 dt 2 a 2 (t) dl 2
The redshift of a galaxy z is defined as z = (λ(t)- λ0)/ λ0 ; (8) where λ0 is the wavelength of a plane wave emitted by the galaxy at the time of emission (the rest wavelength), and λ(t) is the wavelength of the plane wave at the present (the observed wavelength). Thus the redshift and the scale factor are directly linked:
z=(λe-λo)/λ0 = λe/λo-1→range of λe→∞/λ0- very non linear and not proportional: for redshift λe/λo>1→ then λe/λo=1 z=0, when λe/λo=2 z=1=c this expression is incapable of ≈ v/c across the range. For redshift – ( fe-Δf) will be linear proportional to v/c over the total range and will not exceed c or z=1.
Only the proper frequency ratio consistent with Law of conservation of Energy =v/c. The motion away reduces the energy of the photons observed.
At low redshifts, the recession velocity of a galaxy is simply given by cz. At high redshifts, this expression clearly breaks down, and one must go to general relativistic generalizations of it.
Agree
3.1 The Variety of Redshift Surveys In this review, we will concentrate on redshift surveys of well-defined samples of galaxies. By well defined, we mean those in which the selection criteria are quantifiable and reproducible (at least in a statistical way), for without this, it is impossible to do quantitative analyses with them. In practice, this usually means that a sample is denied as limited by some photometric property, usually received flux or diameter in some band. A sample may also have secondary selection criteria as well, such as galaxy morphology, color, or surface brightness. Unfortunately, the data one has available to dene a sample are rarely of very high photometric accuracy, and thus the limits are always approximate to some extent. A redshift survey sample is thus determined by several factors: (i) The region of sky covered. (ii) The photometric quantities with respect to which the sample is denied, and the errors thereof. (iii) The limits on these quantities. (iv) The fraction of the galaxies meeting the selection criteria for which redshifts are measured. From these parameters, several other characteristics of the survey follow, including the total number of galaxies included, the number density of objects surveyed (i.e., its sparseness), and some measure of a typical redshift (the \depth”) in the survey
Cherry picking? It should be random to start.
Redshifts are measured, by necessity, on a telescope attached to the Earth. The Earth takes part in many motions: it is rotating on its own axis (0.3 km s1 at the equator), and it is orbiting around the Sun (30 km s1 ). For extragalactic work, the former correction is negligible, but redshifts are usually published with the correction to the heliocentric frame. However, the Sun is in orbit around the center of the Milky Way (225 km s1 ), the Milky Way is falling towards our nearest large companion, M31 at 119 km s1 (Binney & Tremaine 1987), and the whole Local Group of galaxies takes part in the larger-scale velocity eld which we will discuss in detail below. Because motions on scales smaller than that of the Local Group are very non-linear, we will not include them in our models, but rather refer to redshifts relative to the barycenter of the Local Group. Estimates for the correction from the heliocentric to Local Group barycentric frame have been given by Yahil, Tammann, & Sandage (1977), de Vaucouleurs, de Vaucouleurs, & Corwin (1976) and Lynden-Bell & Lahav (1988). These three determinations are consistent with one another; for example, Yahil et al. quote the motion of the sun relative to the barycenter as 307 km s1 towards Galactic coordinates l = 105, b = 7.
Cherry picking?
6.4 Statistical Bias and Methods of Peculiar Velocity Analysis With the possible exception of the SBF technique, the DIs used in peculiar velocity surveys are not very accurate. The TF and DI- relations, for example, predict galaxy distances with only 20% accuracy. In the volume within which we hope to study the peculiar velocity field in reasonable detail, typical galaxies may lie at distances of 3000 km s1 . The rms peculiar velocity error for such a galaxy is thus on the order of 600 km s1 , which also happens to be the amplitude of typical peculiar velocities. At interesting distances, then, we cannot measure with any precision the peculiar velocity of a single galaxy. Meaningful analyses must use statistical techniques applied to large samples. At the first glance, this might not be considered a major problem. There are, after all, thousands of galaxies in the local volume for which we have obtained, or soon hope to obtain, distance indicator data. We might expect that by virtue of p N statistics alone the signal-to-noise ratio of any statistical analysis could be made rather high. However, this expectation is not realized in practice. The analysis of DI data is instead subject to statistical bias; these result in random errors dropping more slowly than 1= p N, and also in the possibility of large systematic errors if the biases are not properly corrected for. While there are a number of bias, all originate in a coupling of the DI scatter with external
Astronaut Microbes Might Survive at Moon’s Pole, NASA Warns One of the primary objectives of space exploration that often gets overlooked is the intense effort that goes into ensuring Earth microbes don’t contaminate planetary objects, also called forward contamination. This is done to prevent contaminating any potential life that might be present could get killed off my Earth microbes and scientists don’t want to make false discoveries….more
-universetoday
Alien Signals May Hide in Unexplored High Radio Frequencies Confirming the existence of extraterrestrial intelligence via radio signals, also called technosignatures, is the cornerstone of radio astronomy. While humanity has been asking since time immemorial whether or not we’re alone in the universe, radio astronomers are at the frontlines trying to tirelessly answer this longstanding question….more
-universetoday
Mars Curiosity Rover Discovers Massive Field of Polygons The surface of Mars is home to some of the most breathtaking and awe-inspiring landscapes in the solar system. This is primarily due to the Red Planet lacking several re-surfacing processes that Earth possesses, including plate tectonics, volcanism, and flowing water. While Mars does have dust storms, this has done little to reshape the planet’s surface, which has remained largely undisturbed for billions of years…more
-universetoday
What Can We Actually Find on an Exoplanet? Part 1: The Atmospheric Fingerprint In 1990, the Voyager 1 spacecraft, then six billion kilometers from Earth and destined to become humanity’s most far-flung emissary, turned around and took a picture of home. The resulting image is now legendary: a haunting portrait of our tiny, fragile world, a single pale blue dot suspended in a sunbeam…..more
-universetoday
What Can We Actually Find on an Exoplanet? Part 2: A Machine to Find Another Earth It’s not hopeless, because the James Webb isn’t the last telescope we’ll ever build. Next up on the docket of space launches is the Nancy Grace Roman Space Telescope, which will mostly run deep-universe cosmology surveys but will also, almost incidentally, pick out millions of exoplanets using a trick called microlensing….more
-universetoday
What Can We Actually Find on an Exoplanet? Part 3: Reading the Face of a Planet Oh, it’s enough. It’s never enough, of course, but it’s enough to justify building this monstrous planet-hunting telescope, because a single pixel gives you far more than you’d think. That dot has a color, maybe even a pale blue one. It has a spectrum. And it changes brightness as the planet spins on its axis and swings through its orbit…more
-universetoday
What Can We Actually Find on an Exoplanet? Part 4: Looking For Us All of that from a single dot. One HWO observation of an exoplanet can reveal what’s in its atmosphere, sketch a rough map of its surface, show where its oceans and continents lie, and even pick up signs of photosynthesis and how life responds to the changing seasons. And we can go bigger. That same dot can tell you whether there’s not just life, but intelligent life….more
-universetoday
Comet 220P McNaught Puts On An Encore Performance It has been a busy month for astronomy. In the midst of an eclipse season bookended by the total solar eclipse on August 12th and the deep partial lunar eclipse coming right up this week on August 28th, an outbound comet hanging high in the dawn sky just refuses to die: 220P/McNaught…more
-space.com
Chance gaze at old Hubble Telescope image leads to discovery that could reveal dark matter’s secrets In late June 2023, on a morning train commuting to work in New York City, David Hendel was scrolling through a recent astronomy paper on his phone when a half-page image caught his eye. The image, captured a year earlier by the Hubble Space Telescope, showed UGC 9050-Dw1, a dwarf galaxy some 115 million light-years from Earth…more
-phys.org
NASA telescope honors Nancy Grace Roman, architect of space-based astronomy The Nancy Grace Roman Space Telescope is set to launch this weekend aboard a SpaceX rocket from Kennedy Space Center in Florida. When it becomes operational early next year, it will transform our view of the universe….more
-BBC
Want to see a star in close-up? Astronomers have produced one of the most detailed images of Betelgeuse we’ve ever seen. Observations of Betelgeuse by the ALMA observatory in 2023 reveal a strange, bumpy atmosphere and a persistent hotspot on the star’s surface…more
-space.com
James Webb Space Telescope observes 72 stars and finds planet formation is a race against time “Gas giants like Jupiter must assemble their massive atmospheres while the disk is still substantial enough to supply them, before winds and jets carry that raw material away into space.” Using the James Webb Space Telescope (JWST), astronomers have studied 72 young, sun-like stars….more
by Rex Parker, PhD director@princetonastronomy.org
Meeting Sept 8 (7:30pm) on Campus. Welcome back to our monthly astronomy club meetings in person (and Zoom) after the summer hiatus. Reacquaint with other members and catch up on ideas and activities for the club this fall. Our long-time venue Peyton Hall is still under reconstruction and Sherrerd Hall, our location in May, is not available on this date. So, on Sept 8 at 7:30pm we have arranged to meet in Louis Simpson Hall room A71, just a stone’s throw from Sherrerd Hall. Parking lots and streetside spots are very close on William Street. Stay tuned to your email the night before for any last-minute announcements. Here’s a map, Simpson Hall is #8 on this chart. Simpson Hall Map
There have been tremendous advances in astrophotography in the past several years, from smart telescopes to harmonic mounts to remote observing. The Sept 8 meeting will delve into the current state of astrophotography as practiced by amateur astronomers. We’ll feature two members who have great experience here, Dr Michael DiMario and Dr Bob Vanderbei. For more on the presentation please see Victor’s article below.
Also at the Sept 8 meeting after the main presentation, we’ll discuss “What you did on your Summer Vacation”. Here is an opportunity to tell others about your great astro escape or other astronomy/science adventures or trips you took this summer. Please send a note to me and Program Chair Victor if you’d like to hold a 5-10 minute slot to relate your escapades.
The Planets Return to our Stage. Solar system watchers have had some sparse evenings this summer, with Jupiter nowhere in sight till the wee hours. But the situation is becoming more enticing. While Venus has been steady in the western sky after sunset at an astonishing brightness (magnitude -4.4) only recently has its phase become a sight worth pulling your telescope out for. Recalling that only planets sun-ward from earth can display a phase in telescopes, the phases of Venus are interesting to study. The illuminated disk of Venus will be ~40%, making a beautiful telescopic crescent as it shines just south of first magnitude Spica in Virgo in early Sept..
And now the observing windows for the gas giant planets are opening up for evening observing. The opposition of Neptune (when it is closest to earth) occurs during late September. Neptune at opposition is still very far away, about 2.7 billion miles or 240 light minutes from earth. Because it is completely invisible to the naked eye (magnitude ~7.8), Neptune is much less viewed and of course much more difficult to see than the other gas giants. It can be estimated that well under 1 in 1000 people (0.1%) in the US have observed Neptune. Can you hunt down its tiny, bluish-green disk using finder charts near the Great Square of Pegasus? The club’s telescopes at our Washington Crossing Observatory have enough magnification and aperture to make it feasible and worthwhile for you to join that 0.1%.
The planetary positions in September from TheSkyX star chart. Neptune is ~4 light hours away (about 2.7 billion miles, 28 astronomical units).
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