Astronomy Midterm 3

2) Component of Galaxy

2) Halo --> Composed almost exclusively of old metal, pour stars.

It takes about 225 million years for our solar system to orbit the milky way galaxy. About how many times has the sun made the trip around the galactic center since it was formed?

20 times.

Cepheid variables allow measurements of agalxy distances to about

25 MPC away.

3) Component of Galaxy

3) Central Buldge --> Mixture of both types of stars.

There is a small source of intense radiation at the center of our galaxy that may have a mass of about

3.7 mill solar masses.

Milky way

30 kpc=100,000 light years across contains 100 billion stars equal to 8000 pc from sun, distance to galactic center.

What is the mass of the black hole that has been detected at the center of the Milky War?

400 million solar masses.

Local group

54 galaxies, including MW, M3IC (Andromeda) & M33

The Big Bang Theory is favored because:

-Cosmological redshift of galaxies -Cosmic microwave background radition -Formation of large scale structure in universe. -Abundance of hydrogen helium

Period luminosity relation

-Relation between pulsation period of a cepheid variable and its absolute brightness.

If the Schwarzschild radius of a one solar mass black hole is 3 kilometers, what is the Schwarzschild radius of a 10 solar mass black hole?

0.2 AU

Where is the sun located in our galaxy

8 kpc from the galactic center and in the galactic disk

The event horizon of a black hole with the mass of the sun is 3km. The supermassive black ole at the center of galaxy NGC 4261 is 400 million solar masses. What is the size of its event horizon.

80 AU.

From the sun, distance to galactic center is

8000 pc

Higher redshift

= Higher distance

Relationship between Schwarzchild radius and event horizon

= s. rad. = distance from singularity of black hole to event horizon

3 Radius for a 12 solar mass star

=3 (12)= 36 km

Which of these is likely progenitor of a Type IA supernova?

A binary, with a 1.3 solar mass white dwarf accreting matter from its main sequence companion.

1) Components of Galaxy

1. Disk --> Contains gas and dust along with metal rich stars

Mass range for neutron stars

1.4 --> 3 solar masses

The milky way contains about

100 billion stars

Cosmological redshift

A lightwave traveling through an expanding universe "stretches", i.e. wavelength increases

How is a pulsar similar to a lighthouse?

A pulsar is a spinning neutron star whose jet points at us once every rotation, much like a lighthouse whose bean we see when it points at us.

What's a pulsar?

A rapidly rotating neutron star, producing breams of electromagnetic radiation that we detect when the beam points at Earth.

What's a planetary nebula?

A shell of gas surrounding a white dwarf.

Abundance of hydrogen helium

About 25% of ordinary matter is helium.

Where with respect to the main sequence would protostars be located in the H-R diagram?

Above and to the right of it.

How does the age of the universe depend on the Hubble constant?

Age is inversely proportional to Hubble constant. I/Ho= time= age of universe.

Which of these statements is true?

All RR Lyrae stars have the same intrinsic luminosity, about 100L.

Hubble's Law

All galaxies (with nearby exceptions( seem to be moving away from us. The redshifts of their motion in correlation with their distance.

Galaxies are flying apart which means

All matter must have been together at some point in the past.

What is the difference between a pulsar and a neutron star?

All pulsars are neutron stars.

Sprial Galaxies

All rotate fast and have a significant amount of gas.

The presence of a supermassive black hole at the center of our Galaxy has been deduced from

All the other 3 answers are correct.

Measurement of the pulsation period

Allows the distance of the star to be determined.

Cosmic Microwave background

As the universe expands from a hot, dense point, the temp of spacetime decrease (temp today = 30k) radiation from this temp is all around us and is called cosmic microwave background.

At what stage in its life does a star pass through the protostar phase?

Before nuclear reactions begin in its core.

If light from a distant star passes close to a massive body, the light beam will

Bend towards the star due to gravity

If light from a distant star passes close to a massive body, the light beam will

Bend towards the star due to gravity.

Leading theory for energy source in an active galactic nucleus

Black hole, surrounded by accretion disk

A reflection nebula can be seen because of

Blue light scattered by dust.

What is the similarity between a nova and an X-Ray burster?

Both are a result of accreting matter falling onto a compact object from a companion.

In variable stars, what is observed to vary?

Brightness/luminosity

Barred spiral galaxy

Bulge has a bar going through it, and spiral arms originate from ends of the bar.

Not only does the central engine of active galaxies and quasars require a black hole

But also an accretion disk of installing matter to provide the radiation energy.

How do astronomers measure distance to cepheid variables?

By measuring period of variable star, we can determine its luminosity. Then by measuring its apparent brightness, we can use the inverse square law to get its distance.

Collisions between galaxies

Cause the gas and dust clouds to collide, leading to rapid star formation.

For what type of object is period luminosity relation used for determining distances?

Cepheids

Schwarzschild Radius

Critical radius at which escape speed from an object equals the speed of light. Every object has this to which object would have to be compressed for it to become a black hole.

One object that is believed to be a black hole in our Galaxy is

Cygnus X-1, a powerful X-ray source.

Most of the mass of the milky way seems to exist in the form of:

Dark matter out in the galactic halo.

What are the differences between degenerate electron pressure and degenerate neutron pressure?

Degenerate electron pressure= electrons lock up the white dwarf. It can support up to 1.4 solar masses. Degenerate neutron pressure= neutrons lock up a neutron star. it can support up to 3 solar masses.

White dwarfs resist gravitational collapse through

Degenerate-electron pressure, where the electrons lock up and prevent the star from further collapse.

From Earth, the view of the Milky Way is a thin band of stars across the night sky. The part of the Milky Way galaxy that is described here is the

Disk

Where are elements like gold, which are heavier than Iron, created?

During supernova explosions.

Infrared and radio telescopes are ideal for studying galactic center because

Dust in the plane of the MW obscures observations at other wavelengths.

A galaxy that appears flat is labeled

E7

A galaxy that appears round is labeled

EO

What do astronomers believe is the result of a merger between two spiral galaxies?

Ellipitical galaxy (collisions destroy the spiral structure and either cast out gas and dust or turn it into stars.

In the formation of our Galaxy, the ________ formed first.

Globular clusters

Which of the following statements about open clusters and globular clusters is false?

Globular clusters appear red because they contain significant amounts of dust as opposed to open clusters which look blue because they don't have any dust.

Space travel near black holes

Gravitational effects are unnoticeable outside a few Schwarzchild radius, at this distance, black holes do not "suck in" material any more than an extended mass would.

What explanation does general relativity provide for gravity?

Gravity is the result for curved spacetime.

What explanation does general relativity provide for gravity?

Gravity is the result of a curved spacetime.

Cepheids that vary slowly

Have long period and high luminosities.

Cepheids that have short periods

Have low luminosities.

Evidence that a supermassive black hole exists in a galaxy

High energy jets of gas are observed coming from galactic center, stars and gas near center of galaxy exhibit rapid orbital velocities in a fairly small region of space.

The top-left region on the Hertzsprung-Russell diagram is the location of

High mass main sequence stars.

How is the length of a star's lifetime related to the mass of the star

Higher mass stars run through their lives faster and have shorter lifetimes.

Hubble's Constant

Ho

Ellipitical galaxies are classified by

How round or flattened they look

Environment of an accreting black hole

If a black hole is rotating, it can generate strong electric and magnetic fields in its immediate vicinity. These fields draw material from the accretion disk around the black hole and accelerate it into oppositely directed jets along black hole's rotation axis.

Event Horizon

Imaginary spherical surface surrounding a collapsing star, with radius equal to the Schwarzchild radius, within which no object can be seen, heard, or known by an outside surface.

Effects of a black hole

In a binary system, the mass from one star may be "captured" by the black hole. This material spirals into the black hole with so much speed that it emits x-ray light- cygnus x-1 = thought to be a black hole binary.

In what conditions or situations are novae believed to occur?

In binary systems with mass being transferred from one star to another.

If you were looking to find the most recently formed stars in the galaxy, where would you look?

In the galactic disk.

Where in space is the most likely location of a globular cluster?

In the milky way halo, orbiting the galactic center in a random orbit perpendicular to the disk.

The most massive stars end all fusion with a core composed of

Iron

When a star's visible light passes through interstellar dust, the light we see

Is dimmed and reddened.

What is the likely final fate of a star that has a core mass of 2.5 solar masses after completing its nuclear fusion burning phases?

It condenses to the point where it is composed completely of neutrons, the degeneracy of which prevents further collapse.

What happens if the mass of a white dwarf is pushed above 1.4 solar masses?

It explodes as a Type Ia supernova.

What is the size of the dark matter halo of the milky way galaxy?

It is about 100 npc across.

In the evolution of massive stars, what is the significance of the temperature 600 million K?

It is the temperature needed for carbon to fuse into heavier elements.

If the sun were to suddenly become a black hole, what would be its gravitational influence on the Earth?

It would stay the same.

What is the significance of the temperature 600,000,000 K in the evolution of stars?

It's the temperature required for carbon to fuse to oxygen.

Dark matter

Makes up about 85% of matter in universe.

A rotation curve, which plots the rotation speed of objects as a function of their distance from the distance from the center of the MW galaxy can determine its

Mass

Type II supernovae show prominent lines of hydrogen in their spectra, whereas hydrogen lines are absent in spectra of Type Ia supernovae? Why is this?

Massive stars contain large amount of hydrogen, whereas white dwarfs are mostly carbon and oxygen.

Explain the statement "spacetime tells matter how we move, and matter tells spacetime how to curve":

Matter move along the curves of spacetime (that arise due to the presence of another mass) and the curvature of spacetime is caused by the presence of mass.

Variable stars, such as Cepheid variables and RR Lyrae stars, are used in what important measurement in astronomy?

Measurement of distances

Rank from earliest to latest, the stages for the least massive of the high mass stars.

Molecular cloud of gas and dust, gravitational collapse of gas/dust cloud, B spectral class main sequence star, red giant, Type II supernova, neutron star.

Rank from earliest to latest the stages for a low mass star with a companion.

Molecular cloud of gas and dust, gravitational collapse of gas/dust cloud, G spectral class main sequence star, Red giant, planetary nebula, white dwarf, nova (or type 1a supernova, nothing)

Rank, from earliest to latest, the stages for a low mass star without a companion.

Molecular cloud of gas and dust, gravitational collapse of gas/dust cloud, G spectral class main sequence star, Red giant, planetary nebula, white dwarf.

Rank from earliest to latest, the stages for the most massive of the high mass stars.

Molecular cloud of gas and dust, gravitational collapse of gas/dust cloud, O spectral class main sequence star, red giant, Type II supernova, black hole.

The main difference between E0 and E7 galaxies is that E7 galaxies are ____ than E0 galaxies.

More elliptical

While examining spectrum of a galaxy you find all hydrogen lines are shifted towards longer wavelengths. This galaxy is

Moving away from us.

Most interstellar clouds are

Much bigger than our solar system

The core collapse phase at the end of the life of a massive star is triggered when

Nuclear fusion has produced a significant amount of iron in its core.

What vent occurs at the END of the protester state of a star's life?

Nuclear reactions begin in its core, converting hydrogen into helium and generating energy.

Newton's theory of gravity explained not only why the apple fell towards Earth, but also how the planets revolved around the sun. However, 250 years later, Einstein's theory of general relativity revolutionized our thinking about what gravity really means. In this context, which of these statements is false.

The precession of Mercury's orbit around the sun was easily explained using Newton's theory of gravity.

The main difference between normal spirals and barred spirals is

The shape of the central bulge.

What are the main features that make clusters of stars useful to astronomers

The stars were all formed at approximately the same time, so their HR diagram reveals their age.

Pulsars are

Spinning neutron stars

Most of new star formation in the galaxy is found in

Spiral aims

Quasars

Star like in appearance but unusual spectral lines (red shifted)

Cepheid Variable

Star whose luminosity varies in a characteristic way, with a rapid rise in brightness followed by a slower decline. Period is related to its luminosity, to determination of this period can be used to obtain estimate of star's distance (inverse savings law).

Some regions along the disk of the Milky Way appear dark because

Stars in that region are hidden by dust.

Variable stars

Stars whose luminosities change markedly over short periods of time.

Orbits

Stellar orbits of the disk move in a plane and in same direction. Orbits in halo and pledge are more random. All orbit the center of galaxy.

What 2 observations allow us to calculate the galaxy's mass?

Sun's orbital velocity and distance from galactic center.

A high-velocity ring or disk of gas just a few parsecs across at the center of the Galaxy suggests that the exact center is occupied by which of the following?

Supermassive black hole occupies the center

Most active galactic nuclei are powered by

Supermassive black holes

Spiral galaxies are classified based on

Texture of spiral arms and relative size of central bulges.

The diameter of a typical neutron star of 1 solar mass is predicted to be approximately

That of the an average city, about 10km.

The size of a white dwarf is typically like that of

The Earth

According to Hubble's Law, as the distance ti galaxies increases:

The apparent recessional velocity increases.

When all the hydrogen in the core of a star has been converted to helium, what is the very next thing that happens?

The core begins to collapse, since radiation is no longer being produced.

Which of the following statements about interstellar clouds is NOT true

The density of dust in the interstellar medium is like that of the air we breathe.

The farther away a galaxy

The faster it is moving away from us.

Which of these is not an active galaxy?

The great spiral galaxy in Andromeda.

The smallest mass that a main-sequence star can have is about 0.08 solar mass. The reason for this is that

The temperature in the core of a contracting protostar of less than 0.08 Mo does not get high enough for nuclear reactions to start.

The key to identifying a black hole candidate in a binary system is that

The unseen companion in the system must have a sufficiently high mass.

Why do elliptical galaxies tend to appear relish in color?

They are composed mostly of old stars that are characteristically red.

Globular Clusters

Tightly bound swarms of old, reddish (pulsating variable) stars. Most reside at great distances from the sun, 3-P distribution of globular clusters map out a gigantic, roughly spherical volume of space. Interpretation that they define the size/shape of our galaxy.

Galactic Year

Time for our solar system to orbit galaxy (225 mill years)

Radio

Type of active galaxy that emits very strongly in radio portion, enourmous to planet of galaxy.

Seyfert

Type of active galaxy, resembles spiral but cores are thousands times more luminous.

The key to identifying a black hole candidate in a binary system is:

Unseen companion in system must have a sufficiently high mass

Nearest and richest galaxy cluster

Virgo cluster (the local supercluster), containing 2000+ galaxies

SC galaxies shape are

Well defined arms, narrow, and smallest central bulges

In which of these reactions does carbon act as a catalyst?

When hydrogen fuses to form helium in stars with masses greater than about 4 solar masses.

The final remanent of the evolution of a sun-like star is a

White dwarf

Redshift

Z= N/O

For stars on the main sequence

a G star is more massive than an M star.

The mass of a supermassive black hole in the center of the milky way galaxy and the total mass of the milky way galaxy are

about 400 million solar masses and 600 billion solar masses, respectively.

Earth's Schwarzschild radius

centimeter, sun's 3 km

Galaxies rotate

fast but have no gas. They don't have spiral arms but have a disk of stars.

The Solar eclipse of 1919 was used as a test for

general theory of relativity

The source of a protostar's heat is

gravitational energy, converted to heat and light as the protostar contracts

In our galaxy, young metal-rich stars are found

in the disk and spiral arms

Complex molecules in space may be found

inside dense dust clouds like the Horsehead Nebula

Cygnus x-1=

leading candidate for an observable binary system

Elipiical Galaxies have

little or no rotation and no gas, no spiral arms, no disk, reddish colors due to predominance of old red stars

The possible presence of a very large amount of unseen ("dark") matter in the halo of our galaxy is deduced from

rotation curve of our galaxy, which indicates higher than expected orbital speeds in the outer regions of the galaxy.

SA galaxies shape are

smooth, broad spiral arms, and largest central bulges.

Where in the Milky Way would you expect most star formation to be taking place?

the disk and spiral arms

The term "event horizon" refers to

the distance from the center of a black hole at which the escape velocity becomes equal to the speed of light

Neutron stars are believed to be associated with

type II supernovae

Active Galaxies

Radiation is called non stellar radiation because most of light does not come from star luminosity, dominated by activity in galactic center.

Elipitical Galaxy stars have

Randomly oriented orbits around the center of the galaxy

What type of star occurs most commonly in our galaxy?

Red dwarfs

Early attempts to explain spectra of quasars were unsuccessful because spectra were

Redistributed by a larger amount than expected

Intemediately wound barred spiral galaxy

SBb

Spiral galaxy with a large central bulge and tightly wrapped arms

Sa

Irregular Galaxies

Small and forming new stars rapidly, closest neighbors are small and large magellanic cloud.

Which of the following is NOT a characteristic of an active galaxy?

Some of them are radio stars and not galaxies.

At which basic regions of galaxy is sun located

Galactic disk

Which of the following clusters of stars is the oldest?

M67, whose HR diagram shows a main sequences turn off a L=3 Lo.

How does energy output from active galactic nuclei differ from energy output from normal galaxies?

Emission of active galactic nuclei is non stellar, whereas emission of a normal galaxy is.

No stellar disk and no gas/dust

Eo

Which of these is not an indicator of the presence of dark matter

Fast rotation speeds at the edges of spiral galaxies

RR Lyrae Variables

Found in globular clusters, pulsation periods less than a day, all about the same luminosity, "horizontal branch" stars that have roughly the same avg. luminosity (100 L).

Cepheids variables

Found throughout galaxy, pulsation period is 1-50 days, average luminosity is related to pulsation period (++).

Use the concept of escape speed to explain why black holes are said to be black?

Light cannot escape a black hole because the escape speed inside the Schwarzchild radius is greater than the speed of light.

Which type of object exhibits rapid pulses that were originally suspected to be communications from extraterrestrials?

Neutron star

Will the sun ever become a black hole?

No because the sun's mass is not greater than 25 solar masses. It will end up as a white dwarf.

Will our Sun ever explode as a nova?

No, it does not exist in a binary system.

Schwarzschild radius

R=2 GM/C^2 G= Gravitational constant C= Speed of light M= Mass of black hole R=3Mkm