Stars and nebulae

Dark absorption lines were discovered in the solar spectrum in 1802, and Joseph Fraunhofer (1787-1826) recorded the locations of about 600 of them. Comparison to spectra emitted by gases and solids in earth laboratories showed that the gaseous outer layer of the sun contains elements well known on earth. The quantum theory developed in the 1920s yields the frequencies of radiation emitted by atoms as well as the probabilities of emission under various conditions of density and temperature. This allowed astronomers to diagnose the conditions in the solar atmosphere and in the atmospheres of other more distant "suns", the stars in the sky.

The light from the surface of our sun does not show us directly what is happening inside it. However, we now know that the energy source of the sun is nuclear fusion, a concept developed and demonstrated by nuclear physicists. Hydrogen nuclei under high pressures and temperatures fuse to become helium nuclei and other heavier elements. Theoretical models of stars that incorporate a nuclear energy source closely match the observed characteristics of real stars. This understanding has been verified by the measurement of neutrino fluxes from the sun as mentioned above; also see below.

In 1862, a faint companion star to Sirius was first seen with the aid of a new, large (0.46-m) telescope. Observations much later showed it to be very compact, about the size of the earth, but about 350 000 times more massive than the earth (about as massive as the sun). It was called a white dwarf. It was thought that the huge inward pull of gravity of such a compact object would prevent it from remaining stable at its observed size. (In a normal star like the sun, the pressure due to the hot gases prevents such a collapse.) In the mid 1920s, the newly developed quantum theory showed that electron degeneracy pressure would support such a star from further collapse, thus providing a physical basis for the existence of white dwarfs. Degeneracy pressure is strictly a quantum mechanical effect for which there is no classical analog.

Nuclear and quantum physics also led to the demonstration in 1939 that an extremely compact neutron star could be a stable state of matter. It would be about as massive as the sun, but ~1000 times smaller than the white dwarf! It would be as dense as the nucleus of an atom and would consist almost solely of neutrons. (The high pressures within the star force the electrons to combine with the protons.) These stars were finally discovered in 1967, first as radio pulsars, and a few years later as x-ray pulsars. The neutron stars are typically spinning and shine in our direction once each rotation, as does a lighthouse beam. Their periods range from about 1 ms to about 1 ks (1000 s). This pulsing of the radiation gave them their name ("pulsars"). Note that, in this case, the underlying theory of neutron stars existed before their discovery, unlike the case of the white dwarf companion of Sirius where the observations spent decades in search of a theory.

These and other developments brought forward a general picture of the lives of stars. They form from the condensation of large gas and dust clouds in the interstellar medium that appear as beautiful colorful nebula such as the Trifid and Orion nebulae (Figs. 5, 6). A newly formed star stops shrinking and stabilizes when it becomes sufficiently dense and massive so that nuclear burning starts in its center.

Figure 1.5. Trifid nebula, a star-formation region of gas and dust. Newly created hot massive stars radiate ultraviolet light that ionizes the surrounding gas which then emits radiation as it recombines. It is at distance 5500 LY and is 15' in angular extent. The image is about 22' square. North is up and east is left. [T. Boroson/NOAO/AURA/NSF]

Figure 1.5. Trifid nebula, a star-formation region of gas and dust. Newly created hot massive stars radiate ultraviolet light that ionizes the surrounding gas which then emits radiation as it recombines. It is at distance 5500 LY and is 15' in angular extent. The image is about 22' square. North is up and east is left. [T. Boroson/NOAO/AURA/NSF]

Stars often form in groups of 10 or more; these groups are seen as open clusters of stars such as the Pleiades or "Seven Sisters" (Fig. 7). The more massive stars burn out quickly, in a few million years. Intermediate-mass stars like our sun live for ~10 billion years, and lower mass stars would live longer than the age of the universe (10-20 billion years).

As the nuclear fuel in the star is expended, the star goes through several phases of size and color changes. It may expel a cloud of gas to become a beautiful planetary nebula. Radiation from the star excites the atoms in the expanding cloud so they fluoresce, as in the Ring nebula (Fig. 8). The star eventually becomes a compact object, a white dwarf or neutron star. The latter may occur by means of a spectacular supernova implosion/explosion to produce the Crab pulsar and nebula as noted above. If the original star is sufficiently massive, it could instead collapse

Figure 1.6. Orion nebula, a star formation region in the sword of Orion. The nebula is 1300 LY distant and of full optical extent 1.5°. Only the northern half is shown here. The famous trapezium stars are in the southern half. North is up and east is left. [Gary Bernstein, Regents U. Michigan, Lucent Technologies]

Figure 1.6. Orion nebula, a star formation region in the sword of Orion. The nebula is 1300 LY distant and of full optical extent 1.5°. Only the northern half is shown here. The famous trapezium stars are in the southern half. North is up and east is left. [Gary Bernstein, Regents U. Michigan, Lucent Technologies]

to become a black hole, an object so dense that a light beam can not escape from its gravitational pull. Observations and theory together point strongly toward the existence of black holes, but there is still room for more definitive evidence.

Galaxies and the universe

The fuzzy, irregular band of diffuse light that extends across the night sky is known as the Milky Way. Astronomers determined that this light consists of a dense collection of many isolated stars. The Milky Way was thus found to be a large "universe" of stars of which the sun is a member. It is called the Galaxy (with capital G )1 after the

1 We generally follow astronomical practice and use "the Galaxy" to describe the Milky Way system of stars. On occasion, we use "(MW) Galaxy" as a reminder where there could be confusion with other "galaxies".

(a) Pleiades (optical)

Figure 1.7. The Pleiades, or "Seven Sisters", an open cluster of ~100 young stars in (a) optical light and (b) x rays. The haze in (a) is blue light scattered by dust in the cluster. The boxes in (b) indicate the positions of the brightest optical stars. The Pleiades are about 400 LY distant and about 2° in angular extent. North is up and east is left. [(a): © Anglo-Australian Obs./ Royal Obs., Edinburgh; photo from UK Schmidt plates by David Malin. (b) T. Preibisch (MPIfR), ROSAT Project, MPE, NASA]

Figure 1.8. The Ring nebula, a planetary nebula. The central star in a late stage of its evolution has ejected gas which it fluoresces. It is about 2300 LY distant and 1.3' in extent. [H. Bond et al, Hubble Heritage Team (STScI/AURA)]

Greek word gala for "milk". It was in 1917 that Harlow Shapley determined the distance to the center of the Galaxy to be ~25 000 LY1 (current value). He did this by measuring the locations and distances of globular clusters (tightly clustered groups of 105 or 106 old stars; Fig. 9), which he realized must surround the center

1 One light year (LY) is the distance light travels in one year in a vacuum. It is not an SI unit, but we choose to use it because of its natural physical meaning. There are several definitions of the year (i.e. Tropical, Julian and Sidereal) which differ slightly in duration, but each is consistent with the conversion, 1.0 LY = 0.946 x

1016 m ^ 1 x 1016 m. We use the symbol "yr" for the generic year of ~365.25 d where "d" is the non-SI unit for the mean solar day which is about equal to 86 400 SI (atomic) seconds. See Chapter 4.

Figure 1.9. The globular cluster, M10. Globular clusters are remnants from the formation of galaxies. There are about 160 associated with the (MW) Galaxy. Each contains 105 to 106 stars and orbits the center of the Galaxy. M10 is about 65 000 LY from the center of the Galaxy. This photo is 26' full width; the cluster is about 69' in diameter. North is up and east is left. [T. Credner & S. Kohle, Hoher List Observatory]

Figure 1.9. The globular cluster, M10. Globular clusters are remnants from the formation of galaxies. There are about 160 associated with the (MW) Galaxy. Each contains 105 to 106 stars and orbits the center of the Galaxy. M10 is about 65 000 LY from the center of the Galaxy. This photo is 26' full width; the cluster is about 69' in diameter. North is up and east is left. [T. Credner & S. Kohle, Hoher List Observatory]

of the Galaxy. The discus-shaped Galaxy has a diameter of roughly 100 000 LY. It contains about 1011 stars.

The nature of certain diffuse nebulae of small angular extent in the sky was hotly debated: were they diffuse clouds of gas within the Galaxy or were they very distant giant galaxies (with lower case g) similar to the Galaxy? Edwin Hubble obtained a distance to the Andromeda nebula (Fig. 10) in 1924 which turned out to be very large - the current value is 2.5 million light years - which placed the nebula well outside the Galaxy. This distance and its apparent angular size on the sky (~3.4°) demonstrated that Andromeda is another huge galaxy like the Milky Way, of size -100 000 LY.

Figure 1.10. Andromeda nebula M31, our sister galaxy, is about 2.0° in angular extent and is distant 2.5 x 106 light years. North is 26° counterclockwise (left) of "up", and east is 90° further ccw, toward the lower left. [Jason Ware]

Galaxies are found out to great distances; more than 1011 galaxies (or precursor galaxy fragments) are in principle detectable with the Hubble Space Telescope. Some have cores that emit intense radiation at many wavebands. These active galactic nuclei (AGN) may be powered by a massive black hole of mass ~108 solar masses. The most luminous of these cores are known as quasars; they can now be detected to great distances, up to ~90% the distance to the "edge of the observable universe", about 12 billion light years distant.

A great theoretical advance was Albert Einstein's general theory of relativity (1916). This provided a dynamical description of motions in space-time that allowed for accelerating (non-inertial) frames of reference. In this context, gravity can be viewed as a distortion of space. One consequence of this theory is that light rays from a distant star should bend as they pass through a gravitational field, i.e., near a star or galaxy. This effect and its magnitude were first measured in 1919 during a solar eclipse; it made Einstein famous. With more powerful telescopes, gravitational lensing is found to be a prevalent phenomenon in the sky. Distant quasars are sometimes seen as double images or as narrow crescents because an intervening galaxy or group of galaxies serves as a gravitational lens. General relativity also predicts the existence of black holes.

Another consequence of Einstein's theory is that the universe as a whole should evolve. It is expected to be expanding and slowing, or instead, it could be contracting

Recession

Constellation Virgo

Ursa Major

Corona Borealis

Bootes

Hydra

Figure 1.11. Images and spectra of distant galaxies (nebulae). The fainter (and hence more distant) objects show the double absorption (dark) H and K lines of ionized calcium shifted to longer and longer wavelengths, i.e., to the red. If the redshift is interpreted as a Doppler shift, this would indicate a correlation between recession velocity and distance (See Chapter 9). More properly stated, the redshifts are due to the effect of an expanding universe on photons from the distant galaxy as they travel to the earth. Redshift is defined as the fractional wavelength shift, z = (Xobs - Ao)Ao where Xobs and k0 are the observed and rest wavelengths respectively. The redshifts are given here in terms of the equivalent velocities. For speeds much less than the speed of light, these are the Doppler velocities, cz. The distances to the galaxies are derived from the redshift assuming a velocity/distance ratio (Hubble constant) of 15.3 km/s per mega light year. [Palomar Observatory/ Caltech]

and speeding up. The analog is a ball in the earth's gravitational field: it rises and slows, or it falls and speeds up; it does not remain motionless. This aspect of the theory was not appreciated until after Edwin Hubble's observations of distant galaxies showed in 1929 that, indeed, galaxies are receding from one another (Fig. 11); the universe is expanding! This expansion is similar to that of a raisin bread baking in an oven; every raisin moves away from its neighbors as the bread rises. In the universe, the galaxies are like the raisins. This discovery opened up the entire field of cosmology: the story of the birth, life, and death of our universe.

Galaxy Distance velocity image (MLY) (km/s)

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