Chapter 33

Galaxy

A large assemblage of stars, interstellar gas, and dust usually categorized by its shape; elliptical, spiral, or irregular

69. What is the relationship between a white dwarf and a nova?

A nova is a thermonuclear explosion that occurs when a white dwarf collects sufficient mass from a very close neighboring star.

H-R diagram (Hertzsprung- Russell diagram)

A plot of luminosity versus surface temperature for stars. When so plotted, stars' positions take the form of a main sequence for average stars, with exotic stars above or below the main sequence.

27. How does the brightness of a quasar compare with that of a large galaxy?

A quasar is extremely energetic and hundreds of times more luminous than a large galaxy.

10. Among stars originating from the main sequence, which are larger: red stars or yellow stars?

A red star is larger than a yellow star.

Neutron star

A small, extremely dense star composed of tightly packed neutrons formed by the welding of protons and electrons.

24. What type of galaxy is the Milky Way?

A spiral galaxy. It is disc shaped with hot, bright stars and spiral arms.

22. If black holes are invisible, what is the evidence for their existence?

When massive stars reach the end of their nuclear resource they undergo collapse. This collapse continues until the star reaches infinite densities due to high rate of rotation. Gravitation near the surface of these shrunken stars is so enormous that even the light can not escape from them. Hence black holes are invisible. But the objects in closer distance experience its presence light, dust, or a spaceship passing to close drawn into it.

21. How does the mass of a star before its collapse compare with the mass of the black hole that it becomes?

A black hole has the same amount of mass after it collapses as before it collapses.

5. Which is hotter: a red star or a blue star?

A blue star.

Nova

An event in which a white dwarf suddenly brightens and appears as a "new" star

Active Galactic Nucleus

Central region of a galaxy in which matter is falling into a super massive black hole and emitting huge amounts of energy

Giant stars

Extremely large stars toward the end of their life spans found above the main-sequence stars on the H-R diagram

7. What is an H-R diagram?

H-R diagram is a plot of the luminosity versus surface temperature of stars

15. Is the lifetime of a high-mass star longer or shorter than that of a low-mass star?

High-massive stars have shorter lifetimes than low-massive stars.

98. Project what human civilization would be like if our Sun were hidden in a dusty part of the galaxy such that no stars were ever visible to us at night.

Human civilization would not have even existed without the Sun. The main source of energy for life is from the sun, plants obtain energy from the sun for photosynthesis which becomes a food for all the humans and the animals. However, assuming that the sun is not a requirement for life or human civilizations would differ immensely. In most of the religions the Sun is considered as God, and is the only aspect of the existence of life. The Sun is the only distinction between light and darkness. Without this distinction, life would be very difficult. With such wide effect on human civilization science and culture would be completely difficult and is unimaginable if the Sun was hidden in a dusty part of the galaxy.

71. What is expected to happen to the Sun in its old age?

In the old age of star, the supply of hydrogen fuel diminishes, gravity overwhelms thermal pressure and the star pulls inward. Temperature rises as the burned out hydrogen core contracts because of gravity. At a certain point, the temperature becomes high enough in the core to launch helium burning- the fusion of helium to carbon. The star then has a structure consisting of concentric shells. Helium fuses to carbon at the star's center while hydrogen fuses to helium in a surrounding shell. Energy output soars, moving the star of the main sequence.

87. What happens to the radial distance of the event horizon as more and more mass falls into a black hole? Please explain.

It has only one property- mass. And this can be precisely determined outside the event horizon. Addition of mass, a charge on a black hole is not lost to the universe and is an additional physical property.

65. If an entire solar mass were to fall into the Sun, what would happen to its color?

It would be darker, and emit the radiations rapidly.

40. Rank the appearance of the North Star in order of increasing height from the horizon as seen from a) Alaska b) Florida c) Vermont

It's the same as South to North- (b) Florida, (c ) Vermont, (a) Alaska

The longest living stars are those of

Low mass

8. Where are the great majority of stars plotted on an H-R diagram?

Most stars are plotted on the band that stretches diagonally across the diagram

How far is a light year?

Nearly 10 trillion km

18. What is the relationship between a neutron star and a pulsar?

Neutron stars provide an explanation for the existence of pulsars

75. Would you expect metals to be more abundant in old stars or in new stars? Defend your answer.

New stars contain greater amounts of metals. The first and the oldest stars contained hydrogen and helium. After supernovae, heavier elements were released into space. Following stars get enriched with these heavier elements.

48. Is there any star bright enough for us to see it on a sunny day?

No, because sky light overwhelms star light

Local Group

Our immediate cluster of galaxies, including the Milky Way, Andromeda, and Triangulum spiral galaxies plus a few dozen smaller elliptical and irregular galaxies.

The star nearest the earth is

The Sun

9. Where does our Sun reside on an H-R diagram?

The Sun resides in main sequence.

57. What does the color of a star tell you about the star?

The color has to do with its surface temperature.

Quasar

The core of a distant galaxy early in its lifespan when its central black hole has not yet swept much matter from its vicinity, leading to a rate of radiation greater than that from entire older galaxies.

Main sequence

The diagonal band of stars on H-R diagram; such stars generate energy by fusing hydrogen to helium.

Light-year

The distance light travels in a year

Supernova

The explosion of a massive star caused by gravitational collapse with the emission of enormous quantities of matter and radiation

55. Why does the Big Dipper change its position in the night sky over the course of the evening, whereas Polaris, the North Star, remains relatively fixed in its position?

The night side of Earth always faces away from the sun. As Earth circles the sun, different parts of the universe are seen in the nighttime sky.

Big Bang

The primordial explosion of space at the beginning of time

Compared to most stellar objects, quasars are more

distant, energetic, puzzling

Rank yellow, red and blue star in order from least to greatest heat

red, yellow, blue

86. A black hole is no more massive than the star from which it collapsed. Why, then, is gravitation so intense near a black hole?

A black hole has the same amount of mass after it collapses as before it collapsed, so the gravitational field in region at and beyond the original star's radius is different in either case. In accord with Newton's law of gravity, and mass at the surface of star, whether object or particle, has weight that depends both on its mass and on the mass of the star. But weight also depends on the distance between the object and the center of the star.

Pulsar

A celestial object (most likely a neutron star) that spins rapidly, sending out short, precisely timed bursts of electromagnetic radiation

Local Supercluster

A cluster of galactic clusters in which our local group resides

White dwarf

A dying star that has collapsed to the size of Earth and is slowly cooling off; located at the lower left of the H-R diagram.

Starburst galaxy

A galaxy in which stars are forming at an unusually fast rate

1. What are constellations?

A group of stars in a certain pattern in a large section of the sky

70. What event marks the birth of a star? When does a star die?

A star is born when a protostar changes to a star, which occurs when the hydrogen nuclei start to fuse to start a thermonuclear reaction whose pressure that keeps the star from collapsing. The star dies when it reaches the point of no longer being able to consume its thermonuclear fuel.

Planetary nebula

An expanding shell of gas ejected from a low mass star during the latter stages of its evolution

Celestial sphere

An imaginary sphere surrounding Earth to which stars are attached

43. Rank these stages of stellar development from earliest to latest: a) white dwarf b) nova c) red giant

As the hydrogen becomes exhausted in a star of average mass like our sun, the core of the star will eventually contract as the pull of gravity overwhelms the outward pressure, to begin helium burning at some point, in which fusion of helium converts helium to carbon. The outward pressure and increased energy output from the nuclear fusion will cause the star to balloon into a red giant star. Eventually the red giant will collapse into a white dwarf star. In the case of white dwarf stars that orbit each other, the capture of hydrogen from one star to another leads eventually to a thermonuclear blast that we see as a nova. In order from earliest to latest: (c ) red giant, (a) white dwarf, (b) nova

20. Why don't we think the Sun will eventually become a black hole?

Black hole is a fate of super giant or massive star. Sun has too little mass to experience such a collapse.

19. What is the relationship between a super giant star and a black hole?

Black hole is the remains of a super giant star that has collapsed into itself. It is very dense and has such an intense gravitational field that light cannot escape from it.

58. Which has the highest surface temperature: a red star, a white star, or a blue star?

Blue star, because it has the lowest wavelengths, therefore producing the most energy.

97. Compare and contrast astronomy and astrology.

Both study the movement of stars and other celestial bodies in the galaxy and universe. Astronomy is dedicated to observing scientific facts and theoretical implications of celestial systems that happen in the universe. It applies the scientific method and continuously updates its knowledge with each discovery such as heliocentric model, Newton's law of gravity, Einstein's general and special relativity, and Hawking's singularity. These theories are based on scientific data and their implications can be tested and falsifiable. It is an advanced technology, for example the invention of space based telescopes and making new discoveries in the universe. It keeps on changing for every moment. Astrology is the ancient pseudoscience which relates the celestial events with physical and mental and social well-being of the humans. Astrology is based on the belief and luck and has no evidence or the scientific proofs on the celestial events. This is non-confirmable anecdotal evidence and has no development over previous years.

66. Why do low-mass stars live longer than high-mass stars?

Cores of massive stars can reach higher temperatures required for the nuclear fusions of the higher elements like carbon, oxygen, and other elements. As this process gets more complex, they take lesser time. Thus, the most massive stars fuse elements in the cores to produce iron in their cores and within short periods of time they explode and up their lives. The massive stars end their lives within a few million years. Low mass stars fuse the fuel in their cores slowly and move to the other stages of the life slowly. First they fuse hydrogen in the core to the helium and then form a carbon core as the result of the helium fusion. The fusion in the low mass stars take a long time to fuse these elements. This is why low mass stars live longer than high mass stars.

2. Why does an observer at a given location see one set of constellations in the winter and a different set of constellations in the summer?

Different constellations are visible during different seasons because the night sky changes from season to season. The night side of earth always faces away from the sun. It faces different parts of the sky, in different parts of its orbit as the earth circles in its yearly cycle around the sun. Due to this changing nighttime sky, an observer at a given location sees one set of constellations in the winter and a different set of constellations in the summer.

11. What process changes a protostar into a full-fledged star?

Due to thermonuclear fusion, when the central temperature of the proto star reaches about 10 million K, some of the hydrogen nuclei fuse to form helium nuclei. This ignition of nuclear fuel marks the change from proto star to a star. Outward moving radiant energy and the gas accompanying it exert an outward pressure on the contracting matter, ultimately becoming strong enough to stop the contraction. Outward radiation and gas pressure balance inward gravitational pressure, resulting in full-fledged star.

85. With respect to stellar evolution what is meant by the statement "the bigger they are, the harder they fall'?

Dying stars with cores greater than 3 or more solar masses collapse so violently that no physical forces are strong enough to heat continued contraction. The bigger they are, the harder they fall the enormous gravitational field about the imploding contraction mass makes explosion impossible collapse leads to the disappearance of star from universe leaving behind a black hole.

4. Is the light-year a measurement of time or distance?

It is used to measure distance. Astronomers measure vast distances between and the stars using light-years.

77. Some stars contain fewer heavy elements than our Sun contains. What does this indicate about the age of such stars relative to the age of our Sun?

The age of a star is revealed by its elemental make up. The first and the oldest star were composed of hydrogen and helium. After supernovae formed heavier elements and manufactured in giant massive stars were thrown into the space. Following stars get enriched in these elements during their formation. Hence, such stars having fewer elements that our sun are comparatively older.

93. From where does the energy of a quasar originate?

The black holes with the mass about one billion times the mass of the sun are the only sources which would provide such an enormous power to the quasars. The center of the quasar is surrounded by the large rotating cloud of gas. As this gas falls into the black hole, it gets heated up to a few million degrees and then emits the thermal radiation. This radiation spans the spectrum and makes the quasar to appear brighter in the visible spectrum and in the X-rays. Quasars spend only a few million years in a highly luminous phase when the material present in them is sufficient to feed the giant black hole present at its center. Therefore, super massive black holes present at the center of the quasars provide energy to a quasar.

Event Horizon

The boundary region of a black hole from which no radiation may escape. Any events within the event horizon are invisible to distant observers

Polaris is always directly overhead at

The north pole

Black-hole singularity

The object of zero radius into which the matter of a black hole is compressed

54. The Big Dipper is sometimes right side up (could hold water) and at other times upside down (could not hold water). What length of time is required for the Dipper to change from one position to the other?

The present position of the Big dipper is temporary. Its position 100,000 years ago as it appears at present and as it will appear in the future, about 100,000 years from now.

What determines if a star becomes a white dwarf, a neutron star, or a black hole?

The principal factor is mass

Black hole

The remains of a giant star that has collapsed upon itself providing a gravitational field so intense that light itself cannot escape

63. In terms of the life cycle of the Sun, explain why life on Earth cannot last forever.

The sun is about half way through its evolution during which fusion reactions in its core fuse hydrogen into helium. The sun gradually changes to red giant and will have maximum radius beyond Earth's orbit. When the sun loses its present mass due to stellar winds, the orbits of the planets will move outward. Earth's fate is uncertain. It may even collapse into the sun.

17. What is the relationship between a supernova and a neutron star?

The super dense neutron core that remains after the supernova is called a neutron star.

Galaxies are aggregations of stars, stellar dust, and gas. Their masses

vary greatly from one galaxy to the next