Astronomy PDF

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These are my astronomy notes from my intro year....

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Astronomy Josh Ostrander

As the material destined to form a solitary star collapses, it develops a dense, central core of gas and dust. This core temporarily stops collapsing when it becomes opaque to its own infrared radiation. This opaque core weighs in with a mass 1/10 our Sun's, and it spans about 1,000 times our Sun's diameter. The gas-dust core contracts slowly for about 3,000 years, accreting matter from overlying layers all the while

Stars appear to shine with a constant light; however, thousands of stars vary in brightness. The brightness that a star appears to have (apparent magnitude) from our perspective here on Earth depends upon its distance from Earth and its actual intrinsic brightness (absolute magnitude.) The behavior of stars that vary in magnitude (brightness) - known as variable stars - can be studied by measuring their changes in brightness over time and plotting the changes on a graph called a light curve. Amateur astronomers around the world observe variable stars and assist professional astronomers by sending their data to variable star organizations, such as the American Association of Variable Star Observers in Cambridge, Massachusetts. The behavior of some variable stars can be observed with the unaided eye or binoculars. Measuring and recording the changes in apparent magnitude and drawing the resulting light curves will allow you to begin to unravel the stories of the often turbulent and always exciting lives of variable stars. The collection and study of variable star data requires the ability to estimate the apparent magnitudes of stars. The two activities that follow will assist you in acquiring the skill of estimating the magnitudes of variable stars.

There are four main classes (as classified by Hubble). 1. Spiral galaxies. o Disk + central bulge. o M51 Whirlpool Galaxy [type Sc]. o M31 Andromeda Galaxy [type Sib]. o M77 in Cetus [type SBp]. o M104 Sombrero Galaxy [type Sa]. o M85 a ``lenticular'' galaxy (on left) -- mostly bulge, a little disk [type S0]. o NGC5866 a lenticular galaxy, mostly bulge [type S0]. 2. Barred spiral galaxies. o Disk + central bulge with bar. o M83 in Hydra [type SBc if you think it has enough of a bar]. o M91 in Virgo Cluster [type SBb]. o M95 [type SBb]. 3. Elliptical galaxies. o All bulge, elliptical shape, no disk; stars but no gas. o M32 dwarf elliptical galaxy, satellite of M31. o M60 giant elliptical galaxy in Virgo Cluster (on right, with NGC4647). o M87 giant elliptical galaxy, the dominant galaxy in Virgo Cluster. 4. Irregular galaxies. o Irregular shape. o M82 .

How did galaxies get that way? 

The simplest explanation is that

If all the gas is made into stars before the gas has time to form a disk, then you get an elliptical galaxy. o If the gas has time to stabilize into a disk before it is all used up, then you get a spiral galaxy. Or perhaps some of the elliptical galaxies are made from merging of other types of galaxies. o Observations of distant galaxies indicate that spiral galaxies were more common in the past than they are today. o So maybe yesterday's spirals are today’s elliptical. This is an active research area. One problem is that if most of the mass in galaxies is unaccounted for, we have a hard time understanding the dynamics of galaxy formation o

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HR DIAGRAM

Quasars

In the 1960s it was observed that certain objects emitting radio waves but thought to be stars had very unusual optical spectra. It was finally realized that the reason the spectra were so unusual is that the lines were Doppler shifted by a very large amount, corresponding to velocities away from us that were significant fractions of the speed of light. The reason that it took some time to come to this conclusion is that, because these objects were thought to be relatively nearby stars, no one had any reason to believe they should be receding from us at such velocities.

Quasars and QSOs These objects were named Quasistellar Radio Sources (meaning "star-like radio sources") which was soon contracted to quasars. Later, it was found that many similar objects did not emit radio waves. These were termed Quasistellar Objects or QSOs. Now, all of these are often termed quasars (Only about 1% of the quasars discovered to date have detectable radio emission). Here are some Hubble Space Telescope quasar images, and the following figure shows the quasar 3C273, which was the first quasar discovered and is also the quasar with the greatest apparent brightness. It will be discussed further below.

The quasar 3C273. Left image shows the quasar and the jet. Right image superposes on this contours of radio frequency intensity. The sharp radial lines from the quasar are optical spike artifacts because of its brightness (Source).

Quasars Are Related to Active Galaxies The quasars were deemed to be strange new phenomena, and initially there was considerable speculation that new laws of physics might have to be invented to account for the amount of energy that they produced. However, subsequent research has shown that the quasars are closely related to the active galaxies that have been studied at closer distances. We now believe quasars and active galaxies to be related phenomena, and that their energy output can be explained using the theory of general relativity. In that sense, the quasars are certainly strange, but perhaps are not completely new phenomena.

Spectrums

Now go to the heart of the matter, to how free-flying radiation interacts with atoms to give us detailed information about stars and other celestial bodies. Send radiation from a hot, incandescent solid through a gas of low density and watch what happens. The electrons that surround an atom have a minimum energy below, which they cannot go (a discovery of "quantum mechanics" made in the early part of the 20th century). The electrons will naturally seek this level. If you move the electrons outward, away from the nucleus, you give them more energy. The electrons can move farther away from the nucleus, increasing their energies, by collisions with other atoms or by the absorption of photons. However, electrons are very specific about what energies they will take. For any given atom or ion only certain specific electron energies are allowed. In addition, the electrons cannot absorb part of a photon, but must absorb all or none of it. As a result, only photons with particular energies, hence wavelengths (or colors) can be absorbed from the flow of passing radiation. Since the electron structures are different for each kind of atom or ion, the photon energies that each kind will absorb are also different.

When we look at the spectrum from the hot source after it has gone through the lowdensity gas, we therefore see narrow gaps at particular wavelengths where the light is diminished or even gone altogether. Because of the way they appear, these gaps are called "absorption lines." Each atom or ion has a unique set of absorption lines. Hydrogen has only four in the visual spectrum (at wavelengths of 6563 A in the red, at 4861 A in the blue, and at 4300 A and 4101 A in the violet), whereas iron has thousands.

Dark hydrogen absorption lines appear against a continuous visual spectrum, the light in the spectrum absorbed by intervening hydrogen atoms. From "Astronomy! A Brief Edition," J. B. Kaler, Addison-Wesley, 1997.

Absorption lines in stellar spectra The deeper you go into a star, the hotter and denser the gas. The lower layers tend to radiate all the colors rather like a hot solid, while the upper layers act something like the low-density gas of the last paragraph through which the radiation passes. Stars are made of the same stuff as found in the Earth (though not in the same proportions), and contain all of nature's chemical elements. As a result, the spectrum of a star displays an extraordinary mixture of absorption lines. Over 100,000 absorption lines are visible in the Sun's spectrum.

The solar spectrum is filled with absorption lines at particular colors or wavelengths, each dark line associated with a particular atom or ion. The pair in the orange, for example, is made by neutral sodium, the trio in the yellow by magnesium. Kitt Peak National Observatory .

Emission lines What goes up must come down. Electrons, like anything else, will attempt to seek their lowest energies. If the electrons gain energy by the absorption of photons, or perhaps by collisions, they must eventually lose it again. They can lose it in collisions or they can, instead of absorbing photons, radiate them. Since the absorption wavelengths are tightly defined, so are the emission wavelengths. If we look at a heated low-density gas WITHOUT looking at a bright source behind it, we will see BRIGHT lines of color at the same spectral wavelengths at which we before saw dark absorptions. For any given atom or ion, the emission spectrum is a simple reversal of the absorption spectrum. Emission lines are easy to produce in the laboratory simply by heating a low-density gas, allowing collisions to kick the electrons to higher energies. The emissions are produced when the electrons drop back down to lower energies. Emission lines are radiated by street lamps (the orange ones radiating sodium lines, the blue ones mercury lines), neon signs, and fluorescent bulbs. Clouds of interstellar gas that are heated and ionized by nearby hot stars also radiate them. Examples are the great Orion Nebula, a cloud of interstellar gas involved with star birth, planetary nebulae, and supernova remnants. Under some circumstances, stars can radiate emission lines too. For example, Mira variables have hydrogen emission lines that are excited by powerful shock waves -- sonic booms -made by the stars' pulsations.

The absorption lines in the Sun and stars can be identified with individual chemical elements or molecular compounds by comparing their positions in the spectrum (their wavelengths) with those observed from pure sources in the laboratory. Some absorptions are very weak, just shallow dips in the spectrum, whereas others are completely black. The "strength" of an absorption line -- the amount of energy removed from the spectrum -- depends on the amount of the particular chemical element in the star causing the line and on the efficiency of absorption. The efficiency is crucial. Hydrogen dominates the Sun, yet absorption lines of ionized calcium dominate the solar spectrum even though there is 440,000 times as much hydrogen as calcium. Hydrogen has a low efficiency of absorption, whereas that of ionized calcium is very high. The efficiency depends on the availability of electrons to move to higher energies and on atomic factors, namely the likelihood of absorption in the presence of a passing photon. The efficiencies depend critically on temperature and can be calculated from theory or measured in the laboratory. Once they are known, we can calculate the abundances of the atoms from the strengths of the absorption lines and therefore calculate the chemical composition of the outer part of a star. Relative absorption line strengths can also be used to find temperatures and densities. Similar rules can be developed to analyze the emission lines radiated by interstellar gas clouds, from which we learn the compositions of the nebulae, including those of the planetary nebulae.

The Sun displays an enormous number of spectrum lines, over three dozen appearing here in a 20 A-wide stretch in the yellow part of the spectrum. Roman numeral "I" stands for the neutral ion of an element, "II" for the once- ionized version. Different lines have different strengths. The ionized iron lines are nearly black, whereas those produced by the much rare elements yttrium (Y), neodymium (Nd), and lanthanum (La) are very weak. E. C. Olson, Mt. Wilson Observatory.

The spectral sequence Because the efficiencies of absorption depend on temperature, so do the appearances of the spectra of the stars. Stellar spectra were first observed in the middle of the 19th century. To the great confusion of the astronomers of the time, most spectra looked nothing like the solar spectrum. Some, like that of Vega, had powerful hydrogen lines, whereas others had none at all and displayed what were later shown to be molecular lines of titanium oxide. It looked as though different stars were made of different elements. As an aid to understanding, astronomers began classifying the spectra, the schemes culminating about 1890 in the one still used today when E.C. Pickering lettered the stars according to the strengths of their hydrogen lines, his assistants Annie Cannon, Antonia Maury, and Williamina Fleming aiding in development and observation. As observation improved, they dropped some letters, rearranged others according to different spectral criteria, and added decimalization. The result was the classic seven-group sequence OBAFGKM. A bit over a century later, as a result of new technologies, astronomers added another two classes whose spectra contained molecules, L and T. About the first thing any astronomer wants to know about a star is its class. The Sun is class G.

Characteristics of the spectral classes

In the modern spectral sequence, OBAFGKMLT, the hydrogen absorption lines weaken in both directions away from class A. Various other absorptions round out the picture. It was noted very early that the spectral sequence in this form correlates with color, ranging from a blue tint for O and B stars to reddish for class M. Since color depends on surface temperature, so must the spectral class. Stars of class T and cool L radiate only in the infrared and are invisible to the eye.

THE SPECTRAL SEQUENCE Class

Spectrum

Color

Temperature

O

Ionized and neutral helium, weakened hydrogen

Bluish

31,000-49,000 K

B

Neutral helium, stronger hydrogen

Blue-white

10,000-31,000 K

A

Strong hydrogen, ionized metals

White

7400-10,000 K

F

Weaker hydrogen, ionized metals

Yellowish white

6000-7400 K

G

Still weaker hydrogen, ionized and neutral metals

Yellowish

5300-6000 K

K

Weak hydrogen, neutral metals

Orange

3900-5300 K

M

Little or no hydrogen, neutral metals, molecules

Reddish

2200-3900 K

L

No hydrogen, metallic hydrides, alkali metals Red-infrared

T

Methane bands

Infrared

1200-2200 K Under 1200 K

The visual colors are actually subtle and as much reflect where most of the light lies in the spectrum as the color a person would actually view. Classes A through G all look rather white to the eye. Decimal subdivisions of the spectral classes go toward lower temperature, for example, A0 lies at the hot end of class A near a temperature of 10,000 K, while A9 is at the cool end near 7200 K. The Sun, with a temperature of 5800 K, is class G2. (The above temperatures are for main sequence dwarfs. Those of other luminosities may differ, especially in classes G and K, where the temperatures of giants and supergiants ar These

are 20 of the more popular galaxies, star clusters, nebula and double stars for viewing in telescopes and binoculars. Object

Description

Constellation

Notes

M81 & M82

galaxy pair

Ursa Major

between the Dippers

Mizar

double star

Ursa Major

handle of Big Dipper

M51

Whirlpool Galaxy

Canes Venatici

near the Big Dipper

M13

globular cluster

Hercules

in the Keystone

M22

globular cluster

Sagittarius

near the Teapot

M4

globular cluster

Scorpius

near Antares

M8

Lagoon Nebula

Sagittarius

near the Teapot

M7

star cluster

Sagittarius

near the Teapot

M27

Dumbbell Nebula

Sagitta

Epilson Lyrae

double-double star

Lyra

M57

Ring Nebula

Lyra

M31

Andromeda Galaxy

Andromeda

M45

Pleiades cluster

Orion

M42

Orion nebula

Orion

M46 & M47

open cluster pair

Monoceros

M41

open cluster

Canis Major

below Sirius

M44

Beehive Cluster

Cancer

naked-eye visible

Albireo

double star

Cygnus

NGC869 & 884

double cluster

Perseus

Algieba

double star

Leo

naked-eye visible

naked-eye visible

Globular Globular clusters are gravitationally bound concentrations of approximately ten thousand to one million stars. They populate the halo or bulge of the Milky Way and other galaxies with a significant concentration toward the Galactic Center. Spectroscopic study of globular clusters shows that they are much lower in heavy element abundance than stars such as the Sun that form in the disks of galaxies. Thus, globular clusters are believed to be very old and formed from an earlier generation of stars (Population II). More recent estimates yield an age of 12 to 20 billion years; the best value for observation is perhaps 14 to 16 billion (see e.g. the discussion at M92). As their age is crucial as a lower limit for the age of our universe, it was subject to vivid and continuous discussion since decades. The age of globular clusters is determined by investigating their H-R diagrams, as discussed in our globular cluster page. The disk stars, by contrast, have evolved through many cycles of starbirth and supernovae, which enrich the heavy element concentration in star-forming clouds and may also trigger their collapse. Our galaxy has about 200 globular clusters, most in highly eccentric orbits that take them far outside the Milky Way. Most other galaxies have globular cluster systems as well, in some cases (e.g., for M87) containing several thousands of globulars!

Open

Open (or galactic) clusters are physically related groups of stars held together by mutual gravitational attraction. They are believed to originate from large cosmic gas/dust clouds in the Milky Way, and to continue to orbit the galaxy through the disk. In many clouds visible as diffuse nebulae star formation takes still place at this moment, so that we can observe the formation of new young open star clusters (composed of young Population I stars). Open clusters populate about the same regions of the Milky Way and other galaxies as diffuse nebulae, notably spiral arms in disk galaxies, and irregular galaxies, and are thus found along the band of the Milky Way in the sky. Most open clusters have only a short life as stellar swarms. As they drift along their orbits, some of their members escape the cluster, due to velocity changes in mutual closer encounters, tidal forces in the galactic gravitational field, and encounters with field stars and interstellar clouds crossing their way. An average open cluster has spread most of its member stars along its path after several 100 million years; only few of them have an age counted by billions of years. The escaped individual stars continue to orbit the Galaxy on their own as field stars: All field stars in our and the external galaxies are thought to have their origin in clusters.

Binary and ...