astronomy 1020-h stellar astronomy spring_2015 day-32

Astronomy 1020-HSpring_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• Solar Rotation Project due Mon. Apr. 27

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; 3:30-5:00; B310

Where do Stars Form? Spirals

Even in the Galactic Center

Where don’t Stars Form?Ellipticals

The interstellar medium: gas and dust between the stars.

Near the Sun, chemical composition is similar. Most is gas; 1 percent is interstellar dust.

The gas is very tenuous: about 1 atom per cubic centimeter.

It emits various kinds of light, depending on its temperature.

Dust is in the form of solid grains.

“Interstellar soot” (iron, silicon, carbon, and more).

Dust blocks visible light from stars, galaxies, etc.: interstellar extinction.

Size of dust particles: large molecules up to 300 nanometers.

Therefore, dust blocks short wavelengths more efficiently.

More red light is let through: interstellar reddening.

Long wavelengths (infrared and radio) penetrate dust.

A star viewed through dust is fainter and redder due to interstellar extinction and reddening.

The position of absorption lines are not affected.

Dust also emits light when it blocks a star. Energy from absorbed light is transferred to

the dust, heating it. Dust grains are typically cool (10–300 K). Therefore, they emit infrared radiation.

Most gas and dust is concentrated in relatively dense interstellar clouds.

The material found between clouds is called intercloud gas.

Some regions can be very hot (106 K). X-rays emitted, but extremely tenuous. The Sun resides in a local bubble of million-

degree gas. Most intercloud gas is 8000 K.

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