General Astronomy

The Interstellar Medium

Credits: Much of this slideset is modified from lectures by Dr. Peter Newbury (UBC)

Interstellar DustDetection:

Temperature:

Density:

Content:

20 --- 500 K

1000 particles per cubic Km

Small bits of carbon, silicates, nanodiamonds, iron. Some water, methane and ammonia ices.

Dark nebula blocks light from background stars

Interstellar DustDetection:

Temperature:

Density:

Content:

Subaru is Japanese for "Star Cluster"

20 --- 500 K

1000 particles per cubic Km

Small bits of carbon, silicates, nanodiamonds, iron. Some water, methane and ammonia ices.

Reflection nebula reflects the light of nearby stars

Interstellar Dust GrainsDetection:

Temperature:

Density:

Content:

Frozen

100 to 1000 grains per cubic Km

Grains of 106 to 109 atoms of carbon, silicates, nanodiamonds, iron.

Interstellar grains block and scatter (redden) starlight

Interstellar Dust Grains

Is there dust that we cannot see?Yes. Quite a bit in fact.

The space between the stars is filled with thin clouds of dust. Its major effect is in what is known as extinction.

When light from distant objects passes through these clouds, it is reddened (just like a sunrise or sunset)

Extinction• Dimming of starlight at all wavelengths.

– Extinction caused by scattering of light out of the line-of-sight--less light reaches us.

• In 1930, R.J. Trumpler plotted the angular diameter of clusters vs. distance to cluster. – Distance found from inverse square law of brightness. – He found a systematic increase of the linear size of the clusters with

distance. – Unreasonable! It would mean that nature had put the Sun at a special

place where the size of the clusters was the smallest. – More reasonable: the Sun is in a typical spot. It's simply that more distant

clusters have more stuff between us and the clusters so that they appear fainter (farther away) than they really are.

Reddening• Extinction depends on wavelength

– Bluer wavelengths are scattered more than redder wavelengths.• Behavior says that the dust size must be about the

wavelength of light– Less blue light reaches us so object appears

redder than it should. • Trumpler showed that a given spectral type

of star becomes increasingly redder with distance.

Interstellar Gas Clouds

• Neutral Hydrogen (H I)• Ionized Hydrogen (H II)• Ultrahot gas• Giant molecular clouds

Neutral Hydrogen (H I)Detection:

Temperature:

Density:

Content:

Radio telescopes. This is the source of the 21cm band.

100 to 6000 K

1 to 50 atoms per cc

Clouds of Hydrogen 3 to 30 lightyears in diameter.

Also C, O, N, CO, CH, CN, H2 Organic Chemicals! 21cm Milkyway

Optical Milkyway

21-Cm Radio Line

A hydrogen atom has a proton and an electron. In addition to mass and charge, they also have a property known as spin. There are two possible states for spin: Up and Down.

That means there are 4 possible configurations. Up/Up, Up/Down, Down/Up and Down/Down (Proton Spin/Electron Spin). The less-energy states are where the spins are opposite. So the act of "jumping" from a higher state (Up/Up or Down/Down) to the lesser will emit a photon at radio wavelengths (21-cm)

Ionized Hydrogen (H II)Detection:

Temperature:

Density:

Content:

Ionized hydrogen is continually capturing and emitting photons. It is mostly the red H line of Hydrogen

Hot. 100,000 K

100 to 10,000 atoms per cc

Glowing clouds of ionized Hydrogen near hot stars

H II Regions• Fluorescence of hydrogen atoms. • Ultraviolet light from hot O & B stars is absorbed by

the Hydrogen gas and re-emitted mostly at visible wavelengths, primarily 6563 Å(red color).• Each UV photon produces a visible photon. O & B

stars only found in regions of star formation (know why?).

• H II region spectra much simpler than star spectra-easier to decipher. • Distribution of H II regions is in spiral pattern. O & B

are spiral tracers also.

Ultrahot Interstellar GasDetection:

Temperature:

Density:

Content:

Ultrahot105 --- 106 K

1000 atoms per cubic meter

Particles, atoms and molecules are fired out into space at 1000s of km/sec; usually fromSupernovae or other eruptives

Ultrahot outflow heats up the surrounding gas

Giant Molecular CloudsDetection:

Temperature:

Density:

Content:

Complex brightline and darkline spectra.

Varies from very cold 10K to very warm 105 to 106 K (depending on nearby stars)

100,000 atoms per cc

Large clouds, 50 to 200 light years across. H, O, C, N, S,C2H5OH (ethanol),HC3N (cyanoacetylene),CH3CHO (acetaldehyde) Building blocks of DNA

Molecular Clouds• Most of the molecules in the ISM are clumped together into

clouds with masses anywhere from just a few solar masses to over a million solar masses with radii ranging from a few pc to over 100 pc.

• Milky Way has about 2.5 billion solar masses of molecular gas with about 70% of it in a ring at 4-8 kpc distance from the center.

• Not much molecular gas at 1-3 kpc distance from center.

• About 15% of total molecular gas mass is located close to galactic center within 1.5 kpc from the center.

• Most of the gas is clumped in the spiral arms within the disk