by Prasad Ganti
“Betelgeuse is 640 million light years away. It is a red giant, and is almost dying”. How do we know this, when it is difficult to find out what is happening in Syria or Russia? After all no one has visited our Sun carrying either a measuring tape or a thermometer. Indirectly measuring the brightness, luminosity and distances of stars and galaxies has been the hallmark of astronomy and astrophysics in the last century or so.
Firstly, it all started with our own planetary system. Ancient Greeks noticed that the ships disappear over the horizon, thereby concluding that the Earth is round. In fact, Eratosthenes found a well in Syene in Southern Egypt, where on June 21, the sun shines vertically all the way down to the bottom. He noticed that this event never happened in Alexandria. He used the difference in shadows cast in these two places on the same day of the year, the distance between Syene and Alexandria which could be measured with a tape, and some geometry to come up with an approximate circumference of the earth.
Aristarchus built on this idea, he said that at the time of half moon, the Earth, the Moon and the Sun must form a right angle triangle. He came up with approximate distance to the Sun. The Moon’s size can be estimated by the time the Moon takes to move through the Earth’s shadow during a lunar eclipse. Aristarchus predicted that Earth spun around its axis every twenty four hours. Ancient Hindus had similar views. But, for the next thousand years, the Church ruled claiming that the Earth was flat and at the center of our planetary system. Plunging Europe into dark ages.
Then came the Polish mathematician Copernicus who put Sun at the center of the model. Tyco Brahe, the Dane, took observational astronomy to its pinnacle using his observatory. Kepler took Tyco’s data and came up with his famous three laws of planetary motion. Galileo then used the newly invented optical telescope to observe the skies and more famously the four moons of Jupiter. Newton stood on the shoulders of these giants and came up with his famous law of gravitation.
Using Kepler’s laws and Newton’s law of gravitation, the distances within our solar system (up to Saturn) have been determined. Also, the obits have been accurately mapped out. Followed by Uranus, Neptune and Pluto in the next hundred years or so.
Next, moving out of our solar system, Cepheid variable stars were discovered. Whose brightness waned and waxed. Henrietta Leavitt found a relationship between the period of fluctuation and apparent brightness by collecting data from a group of Cepheids in the Magellanic cloud. A team of astronomers found the distance to one Cepheid. Henrietta’s theory was used to calculate the distances of other Cepheids.
Edwin Hubble had been the first to find Cepheids outside the Milky Way and thereby measure the distance to another Galaxy, namely the Andromeda Galaxy. Finding Cephids in distant galaxies was not possible. They are too faint. Astronomers made an assumption that the brightest star in all the galaxies have the same absolute brightness. By comparing the apparent brightness, a Galaxy’s distance could be measured. These stars are known as standard candles.
Another example of a standard candle is the Supernova. Type 1 supernova occurs when two medium sized stars collapse into each other. The resulting fireworks is uniformly luminous regardless of the galaxy in which it occurs. Type 2 supernova results from a massive star collapsing and forming a neutron star or a black hole. This type is not a good standard candle. Type 1 supernovae are used to measure distances to far off galaxies.
Most of the concepts summarized in this article have been borrowed from the book “The Big Bang” by Simon Singh. An excellent history of astronomy, astrophysics, and cosmology. So, we don’t really need a tape to measure larger distances!