How We Know About the Universe

At the various astonomy lectures I've gone too I see different people try to explain the science behind things we know about stars and the universe to varying degrees of success. I figured I'd take my shot at it, with the help of Wikipedia. I'm hoping this helps the layman (and is correct).

You've all seen a prism. Issac Newton did this first in the 1670s and we know now that regular light is made up of component colors. Colors are just different frequencies of light that our eyes can detect differently. There are other frequencies of light like infrared, x-ray and radiowaves that our eyes can't detect but we can build machines that detect them. The component colors are called the spectrum and it's the same colors as the rainbow (water vapor in the air acts as prisms to make rainbows).

In the early 1800s two scientists independently discovered when looking at spectrums from the sun that there were some dark lines in the colors seen. These dark lines are called Fraunhofer lines after one of those scientists, Joseph von Fraunhofer.



In the mid 1800s scientists were discovering and cataloging the different chemical elements, (the Periodic table was invented in 1869). Around this time, scientists discovered that if you heat a substance until it glows and look at the spectrums produced from that light, that different substances showed unique Fraunhofer lines. Therefore, by looking at the spectrum you could determine the chemical makeup of the source. We understand now that this is related to photons energizing electrons in the elements but I won't go into that detail here.

It's important to note that a substance won't be indicated by one just dark line. In fact, a substance is indicated by several lines, in specific positions (ie at specific frequencies) and of different widths. Scientists were looking at the spectrum of stars and categorizing them for a while but it was Cecilia Payne in the 1920s that related the spectra to the temperature and chemical makeup of stars. For this, she earned the first PhD in Astronomy from Harvard. She found that stars (and in fact the universe) consist mostly of Hydrogen and the remainder being almost entirely Helium, but that's the topic of another post.

So lets shift to a different topic. How do we know how far away stars are? For stars relatively close to us scientists starting using parallax around 1830. Hold your finger up to your face and look at it with one eye and then the other. Notice how it's position seems to change against the background. Scientists do this with stars against their background but instead of using the distance of your eyes (a couple of inches) they use the diameter of Earth's orbit. If you look at stars throughout the year and study how they are positioned, you can do geometry to figure out the distance to these close stars. Stars that are further away, however don't seem to move, so you have to use another method to determine their distance.

If they don't move, how do you tell the difference between a bright star far away and a dim star close to us? Wouldn't they both look the same? What scientists use is known as a standard candle, that is something that we know shines at the same luminosity no matter how far away it is. Luminosity is how bright something actually is as compared to how bright it appears because of distance.

If you look at different stars, some are bright, some are dim, some are different colors. Some stars vary how they look (size, brightness) and are called variable stars. Cepheids are a specific type of variable star that are bright (1,000-10,000 times as luminous as the Sun) and pulse over the course of tens of days. The length of this pulse is known as its period. The North Star is a Cepheid variable star.

In 1908 Henrietta Swan Leavitt published her first paper that showed that Cepheids have a pattern that brighter ones have longer periods. Scientists knew the distance of nearby Cepheids by using the parallax method. Studying those, Leavitt could figure out a formula for how much light they produced (their luminousity) based on their period. Now by looking at distant variable stars and measuring their periods, scientists could compare their brightness (appearance) against their luminosity and figure out how far away they were.

In the early 1920s, Edwin Hubble used Cepheids to measure the distance to several nearby "nebulae". Well at the time they were thought to be nebulae but Hubble found they were much further away from other objects. He found some stars were (relatively) close and other objects were very far away and this is how we learned that there are different galaxies from our own (the Milky Way).

So we can figure out how far away things (stars and galaxies) are and we can analyse their spectrum and figure out what they are made of. There's another big discovery here. It turns out when you look at the spectra of different objects, they don't line up exactly the way we think they should. This is where the fact that substances show several different Fraunhofer lines is important. If there was just one line and you saw a line in a different location you'd assume it was different. But scientists saw the set of lines in proper relationship to each other but in the wrong position on the spectrum. They were all shifted to the red side a little bit.

The reason is related to the Doppler Effect, which is the same thing in sound waves. When a train approaches you, it compresses the sound waves making the train sound higher pitched and as it passes the sound waves elongate, making the pitch lower. When light waves shift to the red (longer wave lengths) it shows the object is moving away from us. The crazy part, is that whatever direction from the Earth we look, everything is redshifted, there's nothing that is blue shifted. This means everything is moving away from us.

Hubble didn't discover this redshift but he did correlate the shifts from several galaxies and in 1929 came up with a formula, (known as Hubble's Law) that more distant objects are moving away from us faster than closer objects are. That is, the redshift of more distant objects is greater than the redshift of closer objects and the differences are in proportion to the distance. The neat thing, which is again something for another post, is that this matched up with Einstein's theory of relativity that said the universe is expanding. Einstein (and others) had the theory and Hubble found the evidence.

So here's the last bit, if the universe is expanding now, it was probably expanding before. So in the past it was smaller. If you keep extrapolating backwards, it was very small, the size of a point and then started expanding; that's the Big Bang. There's a lot more to the theory than just this extrapolation and there's even evidence backing it up. For more on the Big Bang, I highly recommend Simon Singh's book Big Bang.

Open questions remain about what will happen to the universe. Will it keep expanding? Since gravity attracts things together will it ultimately stop expanding reach equilibrium or even start contracting?

There's obviously a lot more to these topics. But I think the basic concepts should be understandable to most everyone and in the last two years they've fascinated me. If you found this interesting please post in the comments, I'll do more in the future.