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| Andromeda (M31), M32 and M110, DSLR on telescope tracking mount. |
Andromeda galaxy and its two smaller cousins captured using an SLR camera and a zoom lens from my backyard.
The swarm of stars that look like the background is actually the foreground, all of them within our Milky Way. While we can’t make out any individual stars in Andromeda here, two astronomers with inputs from a third, studied it and happened to change our understanding of the universe and our place in it forever.
How big and how far
One of the great challenges in astronomy has been measuring distance. This is required to understand if for example, we are looking at full size cars on a highway from an airplane or miniature replicas at arms length. Astronomers have devised clever ways to measure distances that work in different situations. For nearer objects they use parallax. An example of parallax is when a finger held at arms length seen by the left and right eyes individually seems to shift. The apparent shift of fixed objects when seen from two different points in space a known distance apart one can compute the distance of the viewed object using simple high school trigonometry. But as distances become very large (hundreds of light years) this method doesn't work well as the "shift" becomes immeasurably small. Hence astronomers use another trick in their bag - the inverse square law of light. This uses the fact that the brightness of a light source is inversely proportional to the square of distance from the observer. It follows that a light source at a given distance, when moved twice as far will be four times dimmer, and three times farther will appear nine times dimmer and so on. Conversely by measuring the amount of dimming of a distant source with known brightness at a prior known distance, one can compute the new distance from the observer. But this works for a source of known brightness, also referred to as a "standard candle" in photometry. Intrinsic brightness is hard to know in astronomy since we are dealing with stars at unknown distances to begin with. Fortunately this problem was solved too, by a Harvard "computer" nominated for the Nobel prize...1908 - Leavitt's Law
At that time computers were people - who computed. And the director of Harvard College Observatory at the time, Edward Pickering had a group of computers - highly educated women working away in what was known as "Edward's harem". Theirs was the lowly routine work of cataloging stars, a job often far beneath their qualification and capabilities. Henrietta Swan-Leavitt was one in the group who would go on to make a major discovery - that would change the scale of the universe forever. Leavitt found that a class of variable stars known as Cepheids blinked or changed brightness and that there was a consistent relationship between their brightness and period of pulsation (time between peaks or troughs of blinking). This came to be known as the period luminosity relationship (sometimes known as Leavitt's law). But what this also meant that if the pulsation period and brightness of one Cepheid at a given distance was known, the absolute brightness of any other Cepheid could be predicted by observing how long it took to "blink". Then using the inverse square law and comparing the apparent brightness to the predicted one, the distance of the second Cepheid could be computed relative to the first. This discovery led George Johnson (author of the book Miss Leavitt’s Stars in 2006) describe her as ‘the woman who discovered how to measure the Universe’. But in her lifetime, Leavitt earned no recognition and remained largely in the shadows and footnotes of articles. Her boss, Pickering published the findings in his own name, mentioning Leavitt as the one who "prepared it". And when a Swedish mathematician nominated her for the Nobel prize for 1926, he had to be informed that she had been dead for four years.
1923 - Hubble's Hoorah
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| Hubble's plate of variable star in M31. Photo Courtesy Carnegie Science Observatories |
The image on the left is Hubble's plate of first Cepheid variable in M31 that he discovered on Oct 5, 1923. It has its letter N crossed out and is marked "VAR!", showing that Hubble originally thought it was a nova, but eventually realized that it varied in brightness like a Cepheid.
Using Leavitt's period-luminosity relationship - he computed its true brightness and hence the distance (using the inverse square law). He concluded that the Cepehid was over 10 times the distance of any other star in the sky. This meant that Andromeda nebula, as it was known then, could not be a cloud inside our galaxy, but must be a distant galaxy itself situated much farther away than anything in our vicinity. There was, hiding in plain sight, a galaxy beyond our galaxy. At this historic point in time, we ceased to be the sole “island universe” . For the first time since our ancestors looked up at the night sky, we came to realize that there were other places like our home galaxy elsewhere.
1970s - Rubin's Revelation
In the 70s, American astronomer Vera Rubin studied orbital speed of stars in Andromeda and concluded that at such spin velocities, there had to be a huge unseen and unaccounted for source of gravity within it to prevent it all from flying apart. This, known as the "galaxy rotation problem" lay the experimental foundation for “dark matter”, which is now believed to be 85% of "mass" in the universe. New York Times described her discovery as a "Copernican Scale" change. |
| Vera Rubin works at the Lowell Observatory in Flagstaff, Ariz., in 1965, Carnegie Institution |
2020 - AMIGA's Amigos
On August 27, 2020; Nasa reported a landmark study involving Andromeda using Hubble's namesake, the Hubble space telescope. In this study, nicknamed AMIGA (Absorption Map of Ionized Gas in Andromeda) scientists have mapped an immense envelope of gas, called a halo (distinct from dark matter halo), surrounding the Andromeda galaxy using 43 bright light sources called quasars in the background. The study suggests that this nearly invisible halo of diffuse plasma extends 1.3 million light-years from the galaxy—about halfway to our Milky Way—and as far as 2 million light-years in some directions. This means while that the Milky way and Andromeda are on a collision course 4 billion years from now, this gaseous halo from Andromeda is already bumping into the halo of our own galaxy in a cosmic handshake. |
| Credits: NASA, ESA, and E. Wheatley (STScI) |
Thanks to efforts of people like Hubble and Rubin, we now know that there are about 100 billion galaxies all around us and these galaxies are embedded in “dark matter halos” that contain most of the “mass” in the universe. What astronomers thought was the universe until the 1900s was merely the tip of a cosmic iceberg.
Nearly a hundred years since Hubble's discovery, Andromeda continues to amaze.
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