Units to cover: 69, 70, 71, 72

Supernovae Type II show prominent lines of hydrogen whereas hydrogen lines are absent in

spectra of supernovae Type I. This is because

• A. White dwarfs consist mostly of hydrogen

• B. Massive stars contain a lot of hydrogen, while white dwarfs are mostly carbon and oxygen

• C. Massive stars have burned all of their hydrogen into heavier elements

• D. None of the above

One of the reasons for pulsars to experience glitches is

A. Nuclear burning in the neutron star

B. Nuclear burning in the white dwarf

C. Cracking of the neutron star crust as it gets more round

D. Cracking of the neutron star crust as it gets more oblate

The first pulsar was discovered by

• A. Sir Edmund Halley, in 1606

• B. Galileo Galilei in 1610

• C. Albert Einstein in 1905

• D. a female graduate, Jocelyn Bell in 1967

The Escape Velocity Limit

• Recall that the velocity necessary to avoid being gravitationally drawn back from an object (the escape velocity) is:

• Also recall that nothing can travel faster than the speed of light, c, or 3108 m/s

R

GMVesc

2=

2

2

c

MGRS

××=

Mass Warps Space

• Mass warps space in its vicinity

• The larger the mass, the bigger “dent” it makes in space

• Objects gravitationally attracted to these objects can be seen as rolling “downhill” towards them

• If the mass is large enough, space can be so warped that objects entering it can never leave – a black hole is formed.

Black Holes

• It takes for a test particle infinite time to fall onto a black hole. How can black holes grow in mass?

Viewing a black hole

• You may be asking, “If light cannot escape a black hole, how can we see one?”

• If a black hole is in orbit around a companion star, the black hole can pull material away from it.

• This material forms an accretion disk outside of the event horizon and heats to high temperatures

• As the gas spirals into the black hole, it emits X-rays, which we can detect!

Light curves from a black hole binary system

General Relativity

• Einstein predicted that not only space would be warped, but time would be affected as well

• The presence of mass slows down the passage of time, so clocks near a black hole will run noticeably slower than clocks more distant

• The warping of space has been demonstrated many times, including by observations of the orbit of Mercury

• The slowing of clocks has been demonstrated as well!

Gravitational Redshift

• Photons traveling away from a massive object will experience a gravitational redshift.– Their frequency will be shifted toward the red end of the

spectrum

If the Sun were replaced by a 1-solar mass black hole, then the Earth would

• A. be destroyed by the gravitational force of the hole

• B. continue to orbit the black hole in precisely its present orbit

• C. spiral quickly into the black hole

• D. head off into interstellar space along a straight line

At what location in the space around a black hole does the escape velocity become equal to the speed

of light?

• A. at the point where escaping x-rays produced

• B. at the singularity

• C. at the event horizon

• D. at the point where clocks are observed to slow down by a factor of 2

Thermal radiation emitted by isolated Black Holes can be understood in terms of

A. Nuclear burning induced by the black hole

B. Expansion of the Universe locally near the black hole

C. Effect of the gravitational field on virtual electron-positron pairs arising due to the quantum mechanical

energy uncertainty

D. Stopping of time in the vicinity of the black hole as the black hole travels through space-time

Star Clusters

• Stars form in large groups out of a single interstellar cloud of gas and dust

• These groups are called star clusters

• Open clusters have a low density of stars – there is lots of space between the cluster’s members

• They can contain up to a few thousand stars in a volume 14 to 40 light years across

• The Pleiades is a very familiar open cluster

Globular Clusters

• Some clusters are much more densely packed than open clusters.

• These globular clusters can have as many as several million stars, in a volume 80 to 320 light years across!

A snapshot of stellar evolution

• Because all stars in a given cluster formed at the same time out of the same cloud of material, we can learn a lot about stellar evolution by examining a cluster’s stars

• We can locate each star in a cluster on an HR diagram and look for the “turnoff point”, the point on the main sequence above which the stars in the cluster have run out of fuel and become red giants

We can deduce the age of a cluster by finding this turnoff point.

Finding a Cluster’s Age

Our Galaxy, the Milky Way

• A galaxy is a large collection of billions of stars

• The galaxy in which the Sun is located is called the Milky Way

• From our vantage point inside the galaxy, the Milky Way looks like a band of stars across the night sky, with dark dust lanes obscuring the center of the band.

An Early View of the Milky Way

• It is difficult to know exactly what the Milky Way looks like from outside the galaxy!– Similar to trying to figure

out what kind of car you are in, from the inside!

• William Herschel (who discovered the planet Uranus) created a “map” of the Milky Way, based on observations.

• He incorrectly placed the Sun close to the center of the galaxy

The Shape of the Milky Way

Kapteyn’s Universe

• Jacobus Kapteyn improved on Herschel’s view of the galaxy

• Using more modern equipment, Kapteyn attempted to count the number of stars in the galaxy, and estimate their distance from the Sun

• The model was called Kapteyn’s Universe, as the existence of other galaxies was unknown!

• He revised the size of the galaxy to around 18,000 parsecs (18 kiloparsecs, or kpc), again with the Sun near the center

• Both Herschel and Kapteyn were correct in depicting the shape of the galaxy as a disk, with most of the stars lying in more or less the same plane (the galactic plane)

Moving the Center of the Galaxy

• Harlow Shapley used observations of globular clusters to correctly deduce the location of the Sun within the Milky Way

• He reasoned that if the Sun were at the center of the galaxy, then globular clusters would be found in all directions

• He noted that there were more globular clusters found in the direction of Sagittarius than elsewhere

• Therefore, the center of the galaxy must be in the vicinity of Sagittarius!

Today’s view of the Milky Way

• Today we know that the Milky Way is a spiral galaxy approximately 30 kps across.

• The Sun is located around 8 kpc from the center, in one of the spiral arms.

• Most of the stars are concentrated in the galactic plane, or in the central bulge at the center of the galaxy

• Inside the bulge is the nucleus of the galaxy

• Surrounding the disk is a roughly spherical distribution of stars called the halo.

• Globular clusters are distributed throughout this halo, surrounding the center of the galaxy.

Two Stellar Populations:Pop I and Pop II

• Astronomer Walter Baade observed an interesting segregation between stars in neighboring galaxies– Younger, blue stars were found mostly in the disks

and spiral arms• He called these “Population I” stars

• Typically less than a few billion years old

• Follow circular orbits in the galactic plane

– Older, red stars were found mostly in the halo and central bulge

• He called these “Population II” stars.

• More than 10 billion years old

• Follow random elliptical orbits around the galactic center – not necessarily in the plane!

• These populations are in the Milky Way, too!

Mapping the Milky Way’s spiral arms

• Once this difference between Population I and II stars was noted, astronomers could map our galaxy’s arms

• Population I stars are mostly bright, blue stars (hot O and B stars) found in the disk

• By measuring the location of O and B stars near the Sun, the first pictures of the Milky Way’s spiral structure were produced.

• Dust and gas obscure the light from more distant stars, so the map is incomplete.

The Formation of the Milky Way

• Our galaxy likely began 13 billion years ago as a huge cloud of pure hydrogen and helium, slowly rotating and collapsing

• The first stars formed within this cloud, burning out quickly and violently. This added heavy elements to the cloud

• Population II stars formed next, capturing some of the heavy elements and settling into elliptical orbits around the center of the cloud

• As the collapse continued, a disk formed, and Population I stars formed from the ashes of dying Pop I stars