Astronomy Exam 3

What is a black hole?

A black hole is a highly collapsed object with such strong gravity that nothing can escape from its vicinity, not even light.

What is a pulsar?

A pulsar is a rapidly rotating neutron star with a strong magnetic field that is beaming radiation along its magnetic axis, which is not aligned with its rotation axis. We see a pulse of radiation when one of the ends of the magnetic axis points at us. This is known as the "lighthouse effect." Most pulsars are detected in the radio part of the electromagnetic spectrum. Some pulsars can also be seen in the optical and X-ray.

Compare the luminosity and surface temperature of a red giant with those for the Sun.

A red giant has a higher luminosity and a lower surface temperature than the Sun

What is the final state of the Sun?

After the Sun ejects its envelope during the planetary nebula phase, the remaining core of the Sun will become a white dwarf. Thus, the final state of the Sun is a white dwarf that gradually becomes cooler and fainter.

Where do the elements heavier than helium originate?

Elements heavier than helium are produced in massive stars. These stars explode in supernova explosions, ejecting these elements (including carbon, nitrogen, and oxygen, on which life is based) into the interstellar medium and thus enriching it. Stars, planets, and life form from gas ejected by supernovae.

How is the energy lost by radiation from the surface of a protostar replaced

Energy radiated from the surface of a protostar is replaced by energy released through gravitational contraction. This keeps a protostar in thermal equilibrium.

What is the maximum possible mass for a white dwarf? For a neutron star?

For a white dwarf, Mmax = 1.4 M . For a neutron star, Mmax ≈ 3 M .

What age stars are found in globular star clusters? What has become of the stars that were formerly on the upper part of the main sequence in these clusters?

Globular star clusters contain only old stars, with ages of about 10 billion years (1010 years). Thus, globular clusters only contain low-mass stars — less than about the mass of the Sun. The higher mass stars that were originally present in globular clusters have already left the main sequence and gone on to red giant and subsequent phases. Thus, these higher mass stars are now red giants, horizontal branch stars, red supergiants, white dwarfs, and neutron stars — depending on their masses. The stars that originally had the highest masses have evolved the furthest from the main sequence phase.

How do stars orbit in the disk of the Galaxy? How do they orbit in the spheroid?

In the disk of the Galaxy, stars follow nearly circular orbits in the same direction. Thus, they have a common orbital shape, common orbital plane, and common orbital direction. The orbits of stars in the disk are like those of planets in the solar system. In the spheroid of the Galaxy (nucleus + bulge + halo), the stars have a much more random orbital pattern. The shapes of the orbits vary from very elongated to nearly circular. There is no common orbital plane nor is there a common orbital direction. The orbits of stars in the spheroid are like those of comets in the solar system.

What is the state of the core of a massive star immediately prior to the star undergoing a supernova explosion? What event triggers the explosion?

Just prior to the supernova explosion, the core of a massive star is composed of iron and it does not undergo nuclear fusion. As the mass of the iron core increases, due to nuclear fusion in the surrounding shells, it eventually exceeds 1.4 M . At this point, the core becomes unstable and collapses. This core collapse is the event that triggers the explosion.

Where do we find low-mass stars on the main sequence? High-mass stars?

Low-mass stars are found in the lower-right part of the main sequence. These stars have low luminosity and low surface temperature. High-mass stars are found in the upper-left part of the main sequence. These stars have high luminosity and high surface temperature.

What is a planetary nebula?

Planetary nebulae are produced in the later stages of life of stars with masses below about 8 times the mass of the Sun. These stars begin unstable pulsations which eject the outer layers of the stars into space where they form expanding gas shells. The stellar remnant left behind will become a small, hot white dwarf. A star may lose more than half its mass in the planetary nebula stage.

Why can more be learned about protostars by observing them in infrared rather than visible light?

Protostars are surrounded by thick dust clouds that absorb more visible than infrared light. Thus more of the infrared radiation from the protostars is able to escape from these clouds, providing us with more information on the nature of the processes within the clouds.

Compare the lifetime of a red dwarf and a blue supergiant. What accounts for this difference in lifetime?

Red dwarf stars have long lifetimes, exceeding 1012 years. Blue supergiant stars have short lifetimes, about 2 × 107 years. This large difference is due to the extremely high luminosity of blue supergiants compared with red dwarfs. This causes blue supergiants to "burn" through their nuclear fuel much faster. Thus, even though blue supergiants have more nuclear fuel (hydro-gen) than red dwarfs, they nevertheless have much shorter lifetimes.

In what types of regions do stars form?

Stars form in cold, dense interstellar clouds. This type of region is known as a dark nebula. The large quantities of dust in a dark nebula absorb the visible light of the stars that lie behind it, blocking them from our view.

Where in the Galaxy is the Sun located?

The Sun is located about 25,000 ly from the center of the Galaxy, in the galactic disk, in a spiral arm.

What is the evidence that the X-ray emitting binary star Cygnus X-1 contains a black hole?

The X-ray emitting binary star system Cygnus X-1 contains a highly collapsed object with a mass of at least 11 M . Since any collapsed object above 3 M is too massive to be supported against gravity as a neutron star, the collapsed object in Cygnus X-1 must be a black hole.

What are the basic components of the Milky Way Galaxy?

The basic components of the Milky Way Galaxy are the nucleus, the bulge, the disk, and the halo.

What is left of a massive star following a supernova explosion?

The collapsed core of a massive star that began with between 8 and 25 M becomes a neutron star, with a mass of about 1.4 M and a radius of about 10 km. This is surrounded by a supernova remnant, a rapidly expanding cloud of gas that once was the envelope of the doomed star. If the initial mass of the star was more than 25 M the final remnant of the star is a black hole.

What type of fusion occurs in the core of a horizontal branch star?

The core of a horizontal branch star undergoes fusion of helium to carbon. Some of the carbon fuses with helium to produce oxygen.

How does the disk of the Milky Way Galaxy appear to us in the sky?

The disk of our galaxy appears to us as a band of diffuse light around the sky, known as the "Milky Way."

What is an event horizon? How is its size related to the mass of the black hole?

The event horizon of a black hole is a spherical surface around the black hole that separates the black hole from the exterior universe. Matter and radiation can pass through the event horizon into the black hole, but nothing can escape from inside the event horizon. Thus, external observers can never see what occurs inside the event horizon. For a 3 M black hole, the radius of the event horizon is 9 km. The radius of the event horizon is proportional to the mass of the black hole, which means that the larger the mass of the black hole the larger the radius of the event horizon.

What are the approximate mass limits for main-sequence stars? What is the reason for these limits.

The lower mass limit for main-sequence stars is 0.08 M . Below this mass, stars do not undergo hydrogen fusion in their cores and are called brown dwarfs. The upper mass limit for main-sequence stars is about 100 M . Above this mass, stars are blown apart by radiation pressure.

What is the significance of the pulse of neutrinos observed from Supernova 1987a?

The neutrinos detected from Supernova 1987a were direct observational evidence that the core of a massive star collapsed, triggering the supernova. It is a confirmation of our theories of supernova production.

Describe in what ways the Sun will change when it becomes a red giant. What will be its energy source during this phase?

When the Sun becomes a red giant, its radius will be larger, its luminosity will be higher, and its surface temperature will be lower than at present. When the Sun becomes a red giant, its energy source will be the fusion of hydrogen to helium in a shell around the core and the gravitational contraction of the core. No nuclear fusion will take place in the core itself when the Sun is a red giant.

Why do we see more stars near the band of the Milky Way than in other directions in the sky?

When we look in the direction of the Milky Way, our line of sight extends a long way through the disk of the Galaxy where the star density is high. When we look away from the Milky Way, our line of sight is more nearly perpendicular to the plane of the Galaxy and it soon reaches regions of low star density. Thus, we see more stars near the band of the Milky Way than in other directions.

Beginning with the main sequence, what are the stages in the life of a 1 M star?

1. Main Sequence 2. Red Giant Phase 3. Helium Flash 4. Horizontal Branch 5. Second Red Giant Phase 6. Planetary Nebula Phase 7. White Dwarf Phase

Draw an evolutionary track in an H-R diagram showing the path of a 1 M star from the main sequence to the red giant phase.

Consult your class notes entitled, "Evolution of a 1 M Star" and Figures 13-1 and 13-3 on pp. 406 and 408 of the eText.

Describe the events that lead to the ejection of the envelope of a massive star in a supernova explosion.

As the iron core of a massive star collapses, the inner part of the envelope follows it inwards. When the core collapses to a radius of about 10 km, it reaches nuclear density and bounces back slightly. This bounce drives a powerful shock wave into the envelope, which pro-vides enough energy to completely eject the envelope into the space around the star.

Describe why a reflection nebula has a bluish color and why a star that lies behind a reflection nebula appears to be redder than its true color.

Blue light is more effectively scattered (reflected) by dust particles than is red light. Thus, we see a blue haze from a cloud of gas and dust that surrounds a star. The source of the blue light is photons from the star that were not originally travelling towards us but that were redirected towards us upon being scattered by the dust in the cloud. Since blue light is more effectively removed from the direct beam of light from the star, the star appears redder than the true color that would be observed if the cloud were not present.

Describe the possible final states of stars. How does the final state of a star depend on its initial mass?

The possible final states of stars are: white dwarf, neutron star, and black hole. The final state depends on the initial mass of the star. If the initial mass of a star is less than about 8 M , the core mass at the end of its life is less than 1.4 M , and the star ends as a white dwarf after the outer layers are ejected in the planetary nebula phase. If the initial mass is greater than 8 M , but less than about 25 M , then the core mass is more than 1.4 M but less than 3 M , and the star ends as a neutron star after the outer layers are blown off in a supernova explosion. If the initial mass exceeds about 25 M , then the core mass exceeds 3 M , and it collapses to a black hole after the outer layers are blown off in a supernova explosion.

What causes space to curve? What effect does spatial curvature have on light rays? What evidence do we have for this effect in the solar system?

The presence of mass in the universe causes space to curve. The curvature is strongest near a very dense mass concentration, particularly a black hole. Spatial curvature causes the paths of light rays to bend. We observe this effect in the solar system, where light rays passing close to the edge of the Sun bend by a small angle as illustrated by Figure 14-5 on p. 439 of the eText. This was predicted by Einstein's General Theory of Relativity in 1916 and was first confirmed by observations during a solar eclipse in 1919.

Explain why Earth's sky is blue.

The sky appears blue since air molecules scatter blue light more effectively than red light. Thus, scattered blue light from the Sun lights the sky. If the Earth did not have an atmosphere, the sky would appear black even when the Sun is above the horizon.

What is the product of helium fusion? When does it begin in a 1 M star?

Three helium nuclei fuse to form a carbon nucleus with a release of energy; this is called the "triple-alpha" process. The mass of the carbon nucleus is slightly less than the total mass of the three helium nuclei; the missing mass is converted into energy according to E = mc2. Helium fusion begins at the upper tip of the red giant phase in a 1 M star when the central temperature of the star reaches 108 K. This stage is called the "helium flash."