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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Stellar burial ground
and cooled – a dynamic environment
And its composition evolves:
Total fraction of heavy elements
Local ISM. Figure
is 500 pc across.
Orange regions indicate
molecular clouds and
sites of intense star
The sun is passing
through a local
cloud piled up by
in turn resides in
a larger low density
cavity called the
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.
the interstellar medium around the sun.
are X-ray sources. Contains several hundred B stars.
Many more low M stars. Stars there are just 5 to 15 My old.
of low density and high
temperature has been inflated
by numerous supernova
explosions. It is about 300
light years long and peanut-
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.
(few tenths solar mass)
about 10 pc
distance to Sirius is 2.7 pc
0.2/cm3 7000 K
Local Bubble - ~100 pc
Very heterogeneous, the boundaries are not sharp
interstellar medium is a clumpy, heterogeneous
place with wide variations in temperature
and density. The galaxy has inflows and outflows.
and H II
Very coarsely defined categories
using a variety of observational techniques
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 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
the Trapezium from the
Hubble Space Telescope.
Note a number of
evaporating disks in nearby
stars – like cometary
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)