The Interstellar Medium and Star Formation
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
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
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
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.
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.
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
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
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.
More on Antares: • 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”.
Sun and the local cloud (few tenths solar mass) about 10 pc distance to Sirius is 2.7 pc
Local Cloud - ~ 5 pc 0.2/cm3 7000 K Local Bubble - ~100 pc 0.005/cm3 106K Very heterogeneous, the boundaries are not sharp
From all this we may correctly infer that the interstellar medium is a clumpy, heterogeneous place with wide variations in temperature and density. The galaxy has inflows and outflows.
Astronomers distinguish four major Mass ~40% ~35% ~20% <5% H I H2 H I and H II H II Very coarsely defined categories
Molecular Hydrogen (H2) • 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.
We study the gaseous component of the ISM 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).
Coronal Gas • 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?
H II Regions • 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
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.
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.
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.
A better resolved image of the Trapezium from the Hubble Space Telescope. Note a number of evaporating disks in nearby stars – like cometary “tails”. John Balley et al (1997)
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)