Astronomy 1020-HSpring_2015

Day-39Stellar Astronomy

Course Announcements

•Final Exam (and Exam-4) is (are) scheduled for Thursday, May 7, 1:30-3:30pm.

• The consensus is: Wed. (5/6) 10:30-12:30: E106B

The unified model of AGN says that the different types of AGNs are partly explained by our viewing angle.

If the AGN’s jets are beamed directly at us, the relativistic beaming of the light produced by the very fast-moving matter overwhelms observations.

CONNECTIONS 20.1

Supermassive Black Holes

The orbital speeds of gas near the black hole yield its mass.

Supermassive black holes probably exist at the centers of all galaxies.

Normal galactic nuclei do not contain accretion disks.

Material in the accretion disk is an AGN’s source of fuel. Without it, the black hole can only be found by gravitational effects.

Supermassive black holes have Schwarzschild radii larger than stellar-mass black holes, but have a smaller density.

The radius of a 6.6-million solar mass black hole is:

MATH TOOLS 20.1

Einstein tells us energy is related to mass by E = mc2.

By measuring the amount of energy emitted by an AGN, assuming an efficiency of matter-to-energy conversion of 10–20%, we can find out how much mass is consumed.

If the efficiency is 15% and the luminosity is 5 x 1035 J/s, then the mass consumed per second is:

MATH TOOLS 20.2

It is unknown what makes a galaxy more suitable for life.

Mergers of galaxies can dramatically change the galactic environment, especially by sparking an AGN and increasing radiation.

This leads to optical illusions of superluminal motion where the jets appear to move faster than the speed of light.

They are not actually violating relativity’s rules.

CONNECTIONS 20.1

Concept Quiz—Blue Light

You do a big survey and find that extremely distant galaxies give off a lot of blue light. What is going on?

A. Interstellar dust is affecting the colors.

B. These galaxies are mainly elliptical galaxies.

C. The galaxies had a high rate of star formation when their light was emitted.

D. You must have made a mistake, since the stars in these galaxies would all be very old.

Wrong ideas can sometimes be useful. Hubble initially created the tuning fork diagram to show his view of galactic evolution, which was wrong.

Astronomers find it useful, however, for classification and teaching.

PROCESS OF SCIENCE

There is a lot of dust in our galaxy. Dust blocks light, making things fainter. They look farther away.

Makes it difficult to find accurate distances to objects that are affected by dust.

The Milky Way is a barred spiral galaxy, as determined by our observations: SBbc.

The starry disk is evident in the night sky in areas with low light pollution.

We can see dark lanes of dust and gas.

Globular clusters can help find distances in our galaxy and to other galaxies.

Bound collections of up to a million stars.

Luminous and can be seen at great distances.

Globular clusters contain very old stars.

Low-mass stars have evolved to be RR Lyrae standard candles.

Shapley used them to find the size of our galaxy and saw that our Sun is not near the center.

Structure of the Milky Way

Globular clusters are spread throughout the spherical halo encompassing the Milky Way disk and central bulge.

The Sun is about 8,300 pc from the center of the Milky Way.

Structure of the Milky Way

Observations of the orbiting speeds of neutral hydrogen gas have revealed a dark matter halo encompassing the rest of the Milky Way, stretching out to a distance of 45,000–50,000 pc from the center.

Doppler shifts from orbital rotation give the speed of gas and stars in the disk.

The neutral hydrogen gas emits radio waves with = 21 cm.

These speed measurements show a flat rotation curve for the Milky Way.

About 90 percent of the matter in the Milky Way is dark matter.

We can apply Newton’s and Kepler’s laws to calculate the mass of the Milky Way inside of the Sun’s orbit.

The Sun’s orbital speed is ~220 km/s, at a distance of 8,300 pc from the center.

MATH TOOLS 21.1MATH TOOLS 21.1

Observing the stars orbiting the Milky Way’s supermassive black hole allows us to calculate its mass.

Star S0-2 has a period of 15.8 years and a semimajor axis of 1.5 x 1011 km.

Use Kepler’s third law, and the fact that the star’s mass is negligible to find the mass:

MATH TOOLS 21.2MATH TOOLS 21.2

Milky Way ScalesLecture Tutorial pg. 135

Work with a partner!Read the instructions and questions carefully.Discuss the concepts and your answers with

one another. Take time to understand it now!!!!Come to a consensus answer you both agree on

and write complete thoughts into your LT.If you get stuck or are not sure of your answer,

ask another group.

Globular clusters in the halo are older than the open cluster stars in the disk.

Globular clusters can be 13 billion years old.

We see no young globular clusters. The processes that formed the two types of

clusters must have been very different.

Stars are mostly hydrogen and helium. Elements heavier than boron must have been

formed in stars. The more massive elements found in a star,

the more prior star formation took place.

New heavy elements are ejected into space through supernovae.

New stars form with some heavy elements. The abundance of heavy elements records

the history of star formation.

We observe younger stars to have more massive elements than older stars.

Globular clusters can have as little as 0.5 percent of the amount of massive elements that disk stars like the Sun possess.

The First Stars The Sun has about a 2 percent

concentration of massive elements. Even globular cluster stars have a small

amount of massive elements. There must have been at least one

generation of stars that existed prior to globular clusters to process those elements.

Those stars would have been very massive, and therefore would have died quickly.

Concept Quiz—Chemical Enrichment

Old stars should have a _____ fraction of heavy elements because _____.

A. large; they have had time to create heavy elements by nucleosynthesis

B. small; chemical enrichment long ago had not enriched the interstellar gas very much

C. small; old stars eject heavy elements into the interstellar medium via mass loss

Younger stars appear close to the thin disk. Thicker disk (~3,700 pc thick) has older stars

that were either captured from galactic mergers or diffused from the thin disk.

High-Velocity Stars Some nearby stars

are members of the halo.

They are on orbits that pass near the Sun, but go far away from the plane.

They have high radial velocities or proper motions.

They are called high-velocity stars.

Concept Quiz—The Sun

Which statement is not evidence that the Sun is a disk star?

A. It has a relatively high fraction of heavy elements.

B. Stars on disk orbits have low velocities compared to the Sun.

C. It is located near the plane of the disk.

Cosmic rays are charged particles moving at near the speed of light.

Can have very high energies. Trapped by the Milky Way’s magnetic field.

There is a black hole at the Milky Way’s center, revealed by its gravity.

Gas orbiting or falling on to the black hole produces X-rays and gamma rays.

About 4 106 M.

Galaxy Formation

The Milky Way offers clues to galaxy formation.

All halo stars have some heavy elements, so at least one prior generation of stars must have existed.

Since halo stars were created from gas, they were formed before interstellar gas was all concentrated into the disk.

Later star formation was all in the disk.

Galaxies are bound gravitationally to others. Smallest are called galaxy groups. The Milky Way is a member of the Local

Group, which has three giant galaxies.

The Milky Way probably formed by the merger of many smaller protogalaxies.

Several of these are still orbiting the Milky Way as satellite galaxies, and the Sagittarius Dwarf has been disrupted, leaving streams of stars.

Andromeda is moving toward the Milky Way.

The galaxies will “collide” in about 4 billion years.

Gravitational disruption and an eventual merger will result.

CONNECTIONS 21.2CONNECTIONS 21.2

Concept Quiz—Young Globulars?

Suppose you found a globular cluster far from the plane that was only 50 million years old. Why would this be surprising?

A. All young objects are in the disk.

B. Globular clusters do not have massive stars.

C. There is little or no gas in the halo.

D. Globular clusters have few heavy elements.

Isaac Asimov’s science fiction novel Nightfall depicts a civilization on a planet orbiting a star that is part of a globular cluster.

In the novel, the planet has six suns and night only comes once every 2,049 years.

That night is a stark contrast to the usually ever-present days, revealing the existence of many, many stars.

How would history have been different if the Solar System had developed in a radically different environment?

CONNECTIONS 21.1CONNECTIONS 21.1

Since the presence of dust was unknown, its effects could not be accounted for.

Once included, more accurate distances were found.

PROCESS OF SCIENCEPROCESS OF SCIENCE