Astronomical Devices

Some of the images in previous lectures were taken through a telescope (and you will see more in the coming weeks). Telescopes are used to make objects both brighter, and bigger. Before we begin our exploration of the more distant reaches of the Universe, we need to take a brief excursion to talk about optics and telescopes. As we noted few weeks ago, Galileo is given most of the credit for being the first to use a telescope for astronomical purposes. But glass lenses have been around since at least 1100 AD, and eyeglasses seem to have been developed in the 13th or 14th century. What good is a telescope? First, let's look at something you are familiar with, your eye:

Your eye has a pupil and a lens.

So, how does a lens work? Well, when light encounters the glass (or some other transparent medium), its speed slows down---light travels more slowly in glass then in air, or in empty space. Thus, if light encounters a tilted piece of glass like that shown below:

it is bent. Think of a marching band trying to march in a line and then executing a turn while trying to remain in a line--those on the outside tip of the line have to walk faster while those on the inside edge of the turn are not moving at all. Now picture a marching band that is in a diagonal line formation marching up a football field, but when they get to the 40 yd line they encounter thick mud. The first band members that hit the mud all of a sudden march more slowly, while those in the dry portion of the field keep marching at the same speed until they hit the mud. This causes a slight bending in the line as they go through the mud. So in glass, the bottom portion of the light beam in the figure above hits the glass surface first, slows down first, and this causes a turn in the beam.

In the eye the lens forms the image (upside down!), while the pupil adjusts in size to allow you to let in more light when it is dark, or less light when it is bright. The size of the pupil is what controls how faint of an object you can see. For example, owls have large pupils to allow in more light since they hunt at night. Cats have sensitive eyes, so when it is bright out, their pupil contracts to a very narrow slit (for more on the nature of the human eye, go here.). The size of the lens is the controlling factor for how faint a light source you can see. That's the first important property of telescopes---they "collect light". Looking through a telescope is like replacing your small eye lens with a much bigger lens. The second important property of a telescope is its ability to magnify objects to see finer detail. The amount of detail that you can see depends on the size of your pupil---or the diameter of the lens of a telescope. This is very similar to parallax. The larger the baseline used in a parallax measurement, the smaller angles (finer details) you can measure. The two extreme edges (diameter) of a lens are your "baseline" for creating detailed images. That is why big telescopes are valuable, they collect light, and allow you to magnify an image to see fine detail.

The amount of light a telescope collects depends on its area. For example, the largest dilated human eye pupil has a diameter of about 8 mm. Therefore, it has an area of 50 square mm (π R2!). A one inch telescope, like Galilo had, has an area of (d= 1 inch = 25mm) 490 square mm. Galileo's telescope collected 10X more light than the human eye, allowing him to see stars that were 10X fainter than those visible with the naked eye. The largest telescope (Keck) has a diameter of 10 meters (10,000 mm), it collects 1.5 million times as much light as the human eye. Here is an analogy, a rain bucket:

During the late 17th and early 18th century, telescope technology was quickly improving. As with Galileo's primitive telescope, nearly all the instruments of that time were "refractors". Refractors use lenses to bend the light to a focus where an eyepiece (similar to a little microscope) magnifies the image:

The problem with early refractors was that they used only a single glass lens, and this arrangement could not bring all colors of light to a common focus. Thus, everything you looked at had blue and red images that were not in focus. This is called chromatic abberation:

To counteract chromatic abberation, the first telescopes were made to be very long. Some of these telescopes had small lenses, on the order of 3" in diameter, but were more than 20 feet long (the longest ones approached 75ft)! This made them difficult to use, and discoveries made with them were rather slow in coming. Though, Christiaan Huygens used one of these primitive telescopes to discover Titan, the largest Moon of Saturn, and to figure out the true nature of Saturn's rings (that's why the probe that entered Titan's atmosphere in 2005 was named "Huygens"). Here is his "big" telescope:

It was discovered later that mixing two (or more) types of glass could counteract the chromatic aberration, and all modern telescopes and camera lenses have more than one lens:

One of the major contributions of Isaac Newton was his book on optics entitled Opticks, published in 1704. One of his proposals was to use a curved mirror to focus light to eliminate chromatic aberration. Newton knew that all light, no matter its color, suffers an identical reflection, thus a telescope using a mirror as its objective would eliminate most of the chromatic abberation suffered by refractors. Using mirrors, telescopes could grow in light gathering ability without becoming too large to manage. One of Newton's designs for such a telescope still bears his name:

Here is a picture of Newton's telescope:

Besides providing better image quality, telescopes with a single curved mirror were easier to produce than refractors (that have two curved surfaces). Thus, they were cheaper. One of the first people to successfully build and use these types of telescopes was William Herschel (1738 - 1822). A drawing of his largest telescope is shown, below.

Herschel is best known for his discovery of Uranus in 1782, a discovery that would give him a secure future as "astronomer to the king". Herschel also discovered two Moons of Saturn, as well as two Moons around Uranus. He also observed binary stars, and showed that their orbits followed Newton's laws, and thus these laws appeared to be "universal" (that is, they worked everywhere in the Universe the same way as on Earth). Currently, the world's largest telescopes are the 10 m Keck telescopes:

Because it is extremely difficult to make a single piece of glass that is 10 m across and optically perfect, the Keck mirrors are segemented:

The next generation of mammoth telescopes are being planned, like the "Thirty Meter Telescope":

The desire for larger and larger telescopes is due to the need to detect the weak, faint objects found in astronomy. But larger isn't all that we need, going into space allows us to get above the distorting atmosphere. Here is an example:

This is why the Hubble Space Telescope was launched:

But techniques are being developed to allow ground based telescopes to reduce the affects of our atmosphere ("adaptive optics"), where a flexible mirror is used to improve the image.

Most cartoons you see have astronomers looking into eyepieces and making drawings. But just about all astronomy now uses digital detectors like those used in your digital camera:

The only difference is we use better quality detectors, bigger detectors (>60 "megapixels"), and chill them to extremely cold temperatures to get rid of electronic noise. But even with simple webcams, you can get amazing quality--here is comparison between a typical snapshot, and an image made by processing a short movie of Saturn I made using a webcam at our campus observatory:

The Milky Way Galaxy

We have been learning about the various kinds of stars, and star clusters that inhabit our galaxy. Now lets talk about our galaxy, the Milky Way. In a very dark location, during most of the year, there is a faint band of light that bisects the night time sky. Because it spans such a large angular size, it is hard to photograph even though it is easy to see. Here is a photograph taken by looking at the reflection of the night sky off a spherical mirror (note the struts that support the camera):

Here is a "fish-eye lens" view of the southern hemisphere sky showing a bright comet (Hyakutake), the fish-eye lens distorts the view, but it is still spectacular:

Close inspection of these pictures shows that the Milky Way does not have a uniform thickness, and is very patchy. In the wintertime sky, the Milky Way is a thin band of light, and is quite faint. In the Summer, however, the Milky Way is brighter, and gets very thick in width towards the constellation of Sagittarius (which is the bright, lower right hand part in the fish-eye view). Note how patchy the Milky Way appears to be---there are dark lanes, or rifts, as well as isolated dark regions. If we photograph the Milky Way throughout the year and construct a composite that represents all 360 degrees of the circle, we get an image that looks like this:

Note that this photograph actually should be printed out and the two ends connected to make a full circle, as the Milky Way appears to be a complete circle around us (this type of map "projection", transforming a sphere into a flat, two dimensional object is also used for world maps). What is the Milky Way, and why does it appear the way it does? This was a puzzle for many years until Galileo pointed his telescope at this faint band of light and found out that it was composed of stars---in fact millions and millions of individual stars. Each by themselves is too faint to see with the naked eye, but when in the company of millions of other faint stars, merge to produce a faint, continous band of glowing light. So, by 1610 or so, we finally discovered that the Milky Way is a band of faint stars. How can we explain its apparent structure? The first model to attempt to explain the Milky Way was made by Englishman Thomas Wright in 1750. He thought that the stars that formed the Milky Way were arranged in a thick ring, and that our solar system was within this ring:

If we look into the ring we see many stars, but if we look perpendicular to the ring, we see few stars. There are some obvious flaws with this model, but it was the first serious attempt to explain what we see. About the same time as Wright proposed his ring model, the philospher Immanuel Kant (1724 - 1804) proposed that the Milky Way was actually a disk, and we were imbedded in the disk. It was left to William Herschel (1738 - 1822) to laboriously count the stars in different directions to figure out which of these models worked best. His results are shown here:

This is a vertical slice through Herschel's model, and the Sun is the large star near the center. Note that to the right, there is a dark lane which has very few stars. This is the same feature shown in the fish-eye view, as that dark lane is towards the direction of Sagittarius. In three dimensions, Herschel's model resembled the disk idea proposed by Immanuel Kant, though it was somewhat lopsided. So what is the cause of these lanes, and dark patches? Dust, of course. If we photograph the Milky Way using infrared light that penetrates right through dust, we get a different view:

Again, this represents a 360o view of the sky, so, you have to imagine wrapping the ends around so they touch, and that you are viewing the picture from inside the cylindrical photograph. In infrared light, the dust clouds are almost transparent (though you still see hints of them as the yellowish areas). Now we can see the true shape of the Milky Way--it is a disk-like object that is thicker in the middle, and gets thinner at the edges. The thickest part of the galaxy is in the direction of Sagittarius, and this is where there are more dust clouds that obscure the light of distant stars causing Herschel's dark finger that lacks stars.

How big is this disk, and what is its structure? This is a very hard question to answer. Imagine standing in the middle of a large forest and then being asked to determine how far the forest extends in each direction! Even in a small grove of trees, you cannot see very far because your line-of-sight eventually intercepts the trunk/branches/leaves of another tree: "Hard to see the forest for the trees". It has taken a long time to figure out the shape and size of our galaxy, but we have done so. In figure 19.1 the size and shape is shown, along with various features of the galaxy:

An enormous amount of information was used to construct this view of our galaxy, including information gathered about other galaxies that seem to have the same structure as ours. For example, we soon found out that all of the stars, star clusters, and molecular clouds in the Milky Way are actually moving! In fact, they are all moving in roughly the same direction--the Milky Way is a rotating disk of stars and gas. The center of this rotation (the point about which all of the stars seem to be moving) is located in the constellation of Sagittarius at a distance of 28,000 light years. Remember that a light year is the distance light travels in a year (see the notes for Class #01 ):

one light year = speed of light x number of seconds in a year

or

1 ly = (3 X 105 km/s) x (3.15 X 107 s/yr) = 9.5 x 1012 km

A light year is thus about 10 trillion kilometers. The center of our galaxy is 28,000 light years from the Sun. The entire disk of the Milky Way has a diameter of approximately 100,000 ly. Note, however, that the disk of stars near the Sun is actually quite thin: about 1,000 ly thick. The thickest part of the Milky Way is called the "bulge". The bulge is a roughly spherical distribution of stars that has a radius of about 5,000 ly surrounding the center of the Milky Way. It is common for astronomers to switch units at this stage. Remember the parsec ("pc")? It was 3.26 ly. To talk about distances within the Milky Way galaxy, it is more useful to talk about them in parsecs, and kiloparsecs (kpc). A kiloparsec is 1,000 parsecs, or 3,260 ly. Using kpc, the Sun is 8.5 kpc from the center, and the Milky Way is 30,000 kpc across (we will soon use the unit Mega-parsec, so get ready!). If we actually plot the locations of all of the young clusters of stars, we find that they actually trace-out the arms of a pin-wheel like structure:

These young hot stars trace out a spiral pattern, and these young stars are located in the "spiral arms" of the Milky Way. While the stars in the spiral arms are generally young (all of the star forming regions are found in the arms and disk), the stars in the bulge of the Milky Way are generally old, red giants. If we could fly high above the disk of the Milky Way, we would expect it to look like this:

Note the pinkish areas--young star formation regions---are mostly confined to the spiral arms There are no star forming regions in the bulge, except in the parts where the disk of the galaxy and the bulge are coincident (that is where they blend together). Note how most of the dust is confined to the spiral arms---where the giant molecular clouds are. In these regions it is cold enough to keep the dust from evaporating. The bulge of our galaxy is older than the disk. Surrounding the galaxy is a low density region called the "halo". There are stars in the halo, but not very many of them, and the ones we find there appear to be very old. Also in the halo are the globular clusters. The globular clusters orbit around the center of our galaxy, and in a way, are like the comets in the Oort cloud orbiting around the Sun:

As we noted last class, the globular clusters are also composed of old stars, even older than the bulge stars.

This gives us one idea of how the Milky Way formed---a view that is almost identical to our model for how the solar system formed: a large spherical cloud of gas and dust collapsed:

The first stars formed out near the edges of this clouds of gas when it was still spherical, and these stars formed the halo. Some of the regions in the cloud of gas were dense enough to form the globular clusters:

As the "proto-galactic" cloud collapses, the small rotation of the cloud is amplified, and this flattens out the cloud forming a disk:

The density was highest in the central regions (like the region surrounding the proto-Sun), so stars formed there before the stars in the disk (which are like the planets), and these stars are older, and there are more red giants there making the bulge have a yellower color than the spiral arms which are full of hot blue stars:

The big difference between the formation of the Sun and solar system and the Milky Way galaxy is one of scale: the gas cloud that formed the Milky Way contained hundreds of billions of solar masses! We know this because we can estimate how many stars there are in our galaxy and that number is between 100 billion and 200 billion.

We believe that the cloud of gas from which the Milky Way formed was intially composed of only hydrogen and helium---no dust or heavier elements. We find that the oldest stars (those in the halo) support this idea, as they seem to have very few elements besides hydrogen and helium. It was the first supernovae explosions that "seeded" the gas clouds with elements like iron, carbon, oxygen, etc. The cycle is shown in Figure 19.2. First we have stars form out of molecular clouds. These stars fuse hydrogen to helium on the main sequence, and then start making carbon and heavier elements as they enter the red giant and supergiant phases. Much of the carbon, oxygen and nitrogen are formed in the atmospheres of red giants, and these elements are returned to the galaxy through the winds of these stars (and during the planetary nebula phases). When massive stars explode, they send the heavier elements out into the galaxy. These elements are then mixed into the molecular clouds, and the next generation of stars has more heavy elements. The youngest stars forming now have more of these elements than the preceding generations. This process is called "chemical enrichment". Without this process, there would be no life on Earth, or even an Earth!

The Heart of the Milky Way Galaxy

Is there anything special about the center of the Milky Way? Unfortunately, we cannot see the center of the Milky Way using visible light---there is just too much dust in the way, and we have to use infrared and radio waves to investigate what is going on there (see Figure 19.23 and 19.24):

When we use these other forms of electromagnetic radiation to examine the center of the galaxy we find that it is a very unusual place. There is a very strong radio source there, and a lot of hot gas that is moving with high speeds in all directions, and swirling around the very center. Something violent seems to have happened there, or is still happening there. When we measure the motions of the stars there we find that they are moving at very high speed in orbits around a small, massive central object:

(Click here for movie showing this motion.) But this object does not appear to emit very much light. A black hole! But unlike the black holes created by the supernovae of massive stars---which have masses of ten times that of the Sun---this black hole appears to have a mass of 10 million solar masses! A "super massive" black hole. Why is it there? The density of stars is very high at the center of the galaxy, and when the massive stars explode forming stellar-sized black holes, there are a large number of black holes orbiting around the center. These black holes could merge to form larger and larger black holes---and then they can further grow in size by eating other stars and gas. We will find that many galaxies like ours have super massive black holes at their centers. We believe that the black hole at the center of our galaxy was once active (and may some day become active again). When the black hole is active, a disk of material--an accretion disk like those in interacting binaries---forms around the black hole, and a jet of material exits along the poles: