The Interstellar Medium
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The Interstellar Medium and Star Formation. The Interstellar Medium. Total mass ~ 0.5 to 1 x 10 10 solar masses of about 5 – 10% of the mass of the Milky Way Galaxy Average density overall about 0.5 atoms/cm 3

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The Interstellar Medium

and

Star Formation


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The Interstellar Medium

  • Total mass ~ 0.5 to 1 x 1010 solar masses of about 5 – 10% of the mass of the Milky Way Galaxy

  • Average density overall about 0.5 atoms/cm3

  • Composition - essentially the same as the surfaces of Population I stars, but the gas may be ionized, neutral, or in molecules (or dust)

  • H I – neutral atomic hydrogen H2 - molecular hydrogen H II – ionized hydrogen He I – neutral helium Carbon, nitrogen, oxygen, dust, molecules, etc.

  • Energy input – starlight, supernovae, cosmic rays

  • Cooling – line radiation and infrared radiation from dust

  • Largely concentrated in (or Galaxy in) spiral arms


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Evolution in the ISM of the Galaxy

Stellar winds

Planetary Nebulae

Supernovae

+ Circulation

Stellar burial ground

The

Big

Bang

The

Interstellar

Medium

Stars

Galaxy

Formation

White Dwarfs

Neutron stars

Black holes

Star Formation


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As a result the ISM is continually stirred, heated,

and cooled – a dynamic environment

And its composition evolves:

0.02

Total fraction of heavy elements

10 billion

years

Time


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500 pc

Local ISM. Figure

is 500 pc across.

Orange regions indicate

molecular clouds and

sites of intense star

formation.

The sun is passing

through a local

cloud piled up by

expansion around

the Sco-Cen

Association. This

in turn resides in

a larger low density

cavity called the

“local bubble”

The Gum Nebula shown in green is a region of hot ionized

hydrogen energized by young stars and the Vela supernova remnant.

The Orion nebula is about 1500 ly away. Not in plane of picture.


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Scorpius-Centaurus Asociation

  • Nearest OB association to sun. Distance 380 to 470 ly Contains many of the bright blue stars in the constellations Scorpius, Antares, and Lupus including as its brightest member Antares - a 15 solar mass star.

  • Includes 1000 - 2000 stars, including most of the brightest stars in the Southern Cross.

  • Several supernovae have happened here in the last 15 My including one - it is thought - 3 My ago that left a deposit of radioactive 60Fe on the Earth.

  • Has blown an evacuated cavity - the “local bubble” in

    the interstellar medium around the sun.


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Sco-Cen Association. Yellow dots

are X-ray sources. Contains several hundred B stars.

Many more low M stars. Stars there are just 5 to 15 My old.

100x100

degrees

ROSAT Survey


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The local bubble, a region

of low density and high

temperature has been inflated

by numerous supernova

explosions. It is about 300

light years long and peanut-

shaped.

600 ly

300 ly


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The bright red supergiant Antares in the lower part of the frame illuminates

a nearby yellowish red emission nebula. The blue (dusty) reflection nebula to the

right contains the star Rho Ophiuchi. Antares may be the next star in the Sco CenAssociation to become a supernova, but it is 600 light years distant.


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More on Antares frame illuminates:

  • Aka Alpha-Scorpii, a magnitude 1.06 star

  • Luminosity 10,000 times that of the sun; Spectral Class M1, T = 3100 K, not a main sequence star (B-V) = 1.83

  • Radius about 4 AU

  • In a binary with a 7 solar mass main sequence star (Type B4) with a period of 878 years. Separation 4 arc sec

  • Name means “Rival to Mars”, sometimes called the “Heart of the Serpent”.


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Sun and the local cloud frame illuminates

(few tenths solar mass)

about 10 pc

distance to Sirius is 2.7 pc


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Local Cloud - ~ 5 pc frame illuminates

0.2/cm3 7000 K

Local Bubble - ~100 pc

0.005/cm3 106K

Very heterogeneous, the boundaries are not sharp


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From all this we may correctly infer that the frame illuminates

interstellar medium is a clumpy, heterogeneous

place with wide variations in temperature

and density. The galaxy has inflows and outflows.


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Astronomers distinguish four major frame illuminates

Mass

~40%

~35%

~20%

<5%

H I

H2

H I

and H II

H II

Very coarsely defined categories


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Molecular Hydrogen (H frame illuminates2)

  • Traced by the radio emission of CO which is found in the same (cold, dense) conditions with a near constant proportionality to molecular hydrogen. H2 itself is not observable in the radio.

  • Mostly concentrated in a ring around the center of the Galaxy

  • interior to the sun’s orbit at 4 to 6 kpc

  • Mostly clumped into clouds ranging in mass from several solar masses to 106 solar masses and sizes from a few ly to 600 ly

  • Molecular clouds contain dust that shields the molecules from destruction by starlight.

  • Some evidence that about 90% of the molecular hydrogen is contained in ~5000 giant cloud complexes with typical masses 105 that of the sun and sizes >65 ly.


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We study the gaseous component of the ISM frame illuminates

using a variety of observational techniques

  • 21 cm to study neutral atomic hydrogen

  • Radio emission from molecules such as CO

  • Optical emission, e.g. H-alpha

  • Ultraviolet emission from hot gas

  • Interstellar absorption lines from cool clouds (optical) and hot medium (ultraviolet).


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Coronal Gas frame illuminates

  • Small fraction of the ISM mass

  • Large fraction of the volume (~50%)

  • Emission lines in the ultraviolet, e.g., O VI

  • Found in vicinity of supernova remnants and above and below disk. Heated by supernova shocks? uv light of O stars?


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H II Regions frame illuminates

  • About 700 in our Galaxy

  • Ionized by intense uv-flux from stars within, especially O stars

  • Detected in radio from high n transitions in hydrogen. Also optical emission in Balmer and Lyman series. Appear reddish, sometime with a greenish tinge from oxygen emission lines. T ~ 104 K. Highly variable density from a few atoms per cc to a million. May contain many stars.

  • Most abundant between 4 to 8 kpc and in spiral arms of the Milky Way. Trace regions of recent star formation.

  • Brightest is less than a million years old.

  • Often have molecular clouds at their boundaries

  • Colorful, but not a major part of the Galaxy’s mass or volume


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Messier 51 frame illuminates


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The Great Nebula in Orion. An illuminated portion of a nearby (1300 ly) giant

molecular cloud. The field of view here is 32 arc min. Each arc min at this distance

is about 0.4 ly.


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The Great Nebula in Orion. A larger field of view. 46 arc min. The inner

regions are glowing in the lines of excited hydrogen which together with

some green from oxygen emission give the inner nebula a yellowish color.

The entire nebula is 29 x 26 light years.


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The Trapezium cluster of stars illuminates the Orion Nebula. These are

the brightest members of a substantial cluster many of whose members lie

hidden behind gas and dust. 8 arc min wide field of view. Distance 1500 light

years.


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A better resolved image of These are

the Trapezium from the

Hubble Space Telescope.

Note a number of

evaporating disks in nearby

stars – like cometary

“tails”.

John Balley et al (1997)


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Lagoon Nebula – in Sagitarius – 5000 ly away – spans 90 x 40 arc min

and 130 by 60 light years. Another H II region on the boundary of a molecular

cloud (like Orion)


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