astronomy 1020 stellar astronomy spring_2015 day-32

Astronomy 1020Spring_2015

Day-32Stellar Astronomy

Course Announcements

• Smartworks Chapter 14: Friday• 2 Dark night observing sessions left:

• Mon. Apr. 13 & Thurs. Apr. 16

• Reports are due Wed. Apr. 22

Astronomy in the Fall, 2015

ASTR-1010/1011 - Planetary Astronomy + Lab (H,R)ASTR-1020/1021 - Stellar Astronomy + Lab (R)ASTR-2010 - Problems in Planet AstronomyASTR-2011 - Intro. to Observational AstronomyASTR-4000/4001 – Astrophotography & LabASTR-4170 – Special Topics in Astronomy

Photometry and Filter SystemsTR; 3:30-5:00pm; B310

H II regions (about 104 K): Hydrogen heated and ionized by ultraviolet

light from hot, luminous stars (O and the hottest B).

Ionized: stripped of one or more electrons.

At lower temperatures, hydrogen is in single, neutral atoms.

This gas emits radio waves with = 21 cm. Light of this wavelength penetrates the dust. Good for mapping the Milky Way.

Many clouds are cold enough for hydrogen to be in the H2 molecule.

These are called molecular clouds. Dense and cold, and appear dark.

Temperatures are around 10 K, with densities as high as 1010 molecules/cm3.

Emit radio waves. Many other molecules are in the mix.

Some clouds can have masses as large as 10 million times that of the Sun: giant molecular clouds.

On average, 120 light-years in size. Stars form in these molecular clouds.

Molecular clouds are cold and dense. Some places in the cloud are denser than

average. Self-gravity will make these regions collapse.

Rate of collapse is slowed by magnetic fields, turbulence, and angular momentum (spin).

Collapse and fragmentation lead to dense star-forming molecular-cloud cores.

The Sun began in one of these cores.

Molecular cores collapse under their own gravity.

Center shrinks fastest; outer layers later.

This produces a dense protostar.

Spin of core produces a disk of material around the protostar.

Material falls onto the growing protostar from the disk.

Our Solar System began this way.

The interstellar material that becomes stars and planets must have the key elements for life if those planets are going to have it.

Water and oxygen have been detected in some star-forming regions.

Concept Quiz—H II Regions

In H II regions, the hydrogen gas is in what form?

A. doubly ionized hydrogenB. once-ionized hydrogenC. neutral hydrogen atomsD. hydrogen molecules

Protostars

Protostars are large, cool, and luminous. They will emit infrared light. Infrared studies of molecular regions reveal

protostars and their disks.

The protostar continues to accrete more material.

It continues to shrink and radiate away energy, balancing pressure and gravity.

The interior temperature and pressure rise.

The low temperature of dust means that it glows in the infrared.

100 K dust:

10 K dust:

MATH TOOLS 15.1MATH TOOLS 15.1

The protostar’s energy source is gravitational energy.

As it shrinks, temperature rises in the core.

Hydrogen fusion begins in the core: It becomes a main sequence star.

The temperature in the core must be hot enough for fusion, 10 million K.

Very low-mass stars (< 0.08 M) never start hydrogen fusion.

These are called brown dwarfs.

Concept Quiz—Energy

What is the source of energy for a protostar on the Hayashi track?

A. hydrogen fusionB. bipolar jetsC. gravitational contractionD. angular momentum

Evolutionary Tracks

An individual star follows an evolutionary track on the Hertzsprung-Russell diagram.

This is the path of the temperature and luminosity with time.

Evolutionary Tracks

Protostars get less luminous (for lower masses), smaller in radius, and hotter.

The star moves on the Hayashi track and arrives on the main sequence.

1,056

A lower-mass star like the Sun is more luminous as a protostar than as a main sequence star, even though it is cooler as a protostar.

This is due to its physical size (radius).

MATH TOOLS 15.2MATH TOOLS 15.2

Concept Quiz—Evolutionary Tracks

Once fusion begins, a star moves to the left on the H-R diagram. Its luminosity does not change, but its temperature rises. The star is

A. expanding.B. contracting.C. staying at the same radius.D. can’t tell from the information given