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Hubble Diagrams and Relative Distances

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REMINDER: All answers go on your edu20.org blog as an entry labeled "Assignment 2"


Assignment 2: Hubble Diagrams


and Relative Distances



How Do We Know?

How do we know the universe is expanding? Well, for a long time, we didn't know. Although the universe is expanding all around us, the expansion happens over such a large scale that it's very hard to notice it from Earth. The expansion of the universe was discovered in 1929 by American astronomer Edwin Hubble. He used the 100-inch telescope on Mount Wilson in California - at the time, the largest telescope in the world - to measure the distances to other galaxies.


He combined his distances with another scientist's measurements of the speed with which galaxies were moving away from us. He created a graph of distance vs. velocity for the universe, just like the graph you created for the balloon. Looking at his graph, he realized that the universe must be expanding. Today, we call such a graph a Hubble Diagram.


The Universe Project

You don't just have to take our word for it, though - you can make your own Hubble Diagram to prove to yourself that the universe is expanding! We will retrace Hubble's steps to make one of the most important discoveries of 20th century astronomy.

We will look at a few galaxies in an database. This database contains actual data that has been collected by many astronomers. It provides astronomers with a catalogue of objects that are known to exist in the universe. So far, there are nearly 90 million stars and galaxies in this catalog! The database is being used by astronomers to make the largest, most detailed map of the universe ever made.

Our first step will be to look at how bright these galaxies are in order to get a rough idea of how far away from Earth they are. Next, we will use these distances, along with SkyServer's measurements of how red the galaxies are, to make a simple Hubble Diagram to show the universe's expansion.



The first step in creating a Hubble diagram is to find the distances to several galaxies. Unfortunately, measuring distances in astronomy is difficult. Fortunately, all you need for a simple Hubble diagram are relative distances to galaxies, not their actual distances measured in light-years or kilometers. In other words, you don't need to know exactly how far away a certain galaxy is.

To measure relative distance, you need some way to compare galaxies. Since most galaxies are generally similar, you can try assuming that they all have the same average properties - that each galaxy is just about as bright and just about as big as any other galaxy.

The picture below gives an idea of what you can conclude when you assume that all galaxies are similar. The picture shows two identical cans at different distances. The nearer can looks bigger than the farther can. If the cans were giving off light, you could imagine that the nearer can would look much brighter than the farther can.


Nearby objects appear larger than distant objects

The same effect occurs with galaxies. When we assume that two galaxies are similar (in brightness and size), then any differences in brightness or size we see between them are due only to their distances from us. Look at the following images:

Nearby galaxies appear large and bright, while
distant galaxies appear small and faint.


We can assume that the galaxy that appears larger is just like the galaxy that appears smaller, but much closer to us.


Question: What are some measurements you can make to tell which galaxies are closer and which galaxies

are farther? How do these measurements relate to the distance to the galaxies?


Magnitudes and Distances


One of the easiest ways to compare galaxies is to compare their magnitudes. Magnitude is a measure of how bright a star or galaxy looks to us - how much light from that star or galaxy reaches Earth. In magnitude, higher numbers correspond to fainter objects, lower numbers to brighter objects; the very brightest objects have negative magnitudes.

The scale is set up so that if object A is 2.51 times fainter than object B, then object A's magnitude will be higher by one number. For example, a magnitude five galaxy is 2.51 times fainter than a magnitude four star. The sun has magnitude -26. The brightest star in the Northern sky, Sirius, has magnitude -1.5. The brightest galaxy is the Andromeda Galaxy, which has magnitude 3.5.

The faintest object you can see with your eyes has a magnitude of about 6. The faintest object the SDSS telescope can see has a magnitude of about 23. SDSS measures magnitudes in five wavelengths of light: ultraviolet (u), green (g), red (r), near infrared (i), and infrared (z).


The image below shows the difference in brightness between a magnitude 16 galaxy and a magnitude 19 galaxy in the SDSS's green (g) wavelength. The magnitude 16 galaxy is (2.51 x 2.51 x 2.51 =) 15.8 times brighter than the magnitude 19 galaxy.


Question: Why can magnitudes be used as a substitute for distances?



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