Chapter 11 The Interstellar Medium

11.1 Interstellar Matter The interstellar medium consists of gas and dust.

11.1 Interstellar Matter The interstellar medium consists of gas and dust. This is part of a giant cloud of gas and dust called the Eagle Nebula

11.1 Interstellar Matter The interstellar medium consists of gas and dust. Interstellar gas consists of atoms and small molecules, mostly hydrogen and helium.

11.1 Interstellar Matter The interstellar medium consists of gas and dust. Interstellar gas consists of atoms and small molecules, mostly hydrogen and helium. The dust is clumps of atoms and molecules; similar to soot or smoke

11.1 Interstellar Matter The interstellar medium consists of gas and dust. Interstellar gas consists of atoms and small molecules, mostly hydrogen and helium. The dust is clumps of atoms and molecules; similar to soot or smoke The dust absorbs and scatters light

11.1 Interstellar Matter The interstellar medium consists of gas and dust. Interstellar gas consists of atoms and small molecules, mostly hydrogen and helium. The dust is clumps of atoms and molecules; similar to soot or smoke The dust absorbs and scatters light The light that does get through is redder

11.1 Interstellar Matter The interstellar medium consists of gas and dust. Interstellar gas consists of atoms and small molecules, mostly hydrogen and helium. The dust is clumps of atoms and molecules; similar to soot or smoke The dust absorbs and scatters light The light that does get through is redder This image shows distinct reddening of stars near the edge of the dust cloud 

11.1 Interstellar Matter Dust clouds scatter blue light preferentially, so the light is reddened, but spectral lines do not shift…

11.1 Interstellar Matter Dust clouds scatter blue light preferentially, so the light is reddened, but spectral lines do not shift…what does this tell you?

11.1 Interstellar Matter Notice how the infrared light penetrates the cloud better than the visible light?

11.1 Interstellar Matter Notice how the infrared light penetrates the cloud better than the visible light? Knowing what you now know about light scattering by these clouds, why does this make sense?

11.2 Star-Forming Regions This is the central section of the Milky Way galaxy, showing several nebulae, areas of star formation.

11.2 Star-Forming Regions These nebulae are very large and have very low density; their size means that their masses are large despite the low density.

11.2 Star-Forming Regions “Nebula” is a general term used for fuzzy objects in the sky. Dark nebula: dust cloud Emission nebula: glows, due to hot stars

11.2 Star-Forming Regions Emission nebulae generally glow red – this is the Hα line of hydrogen. The dust lanes visible in the previous image are part of the nebula, and are not due to intervening clouds.

11.2 Star-Forming Regions How nebulae work:

11.2 Star-Forming Regions There is a strong interaction between the nebula and the stars within it; the fuzzy areas near the pillars are due to photoevaporation:

11.2 Star-Forming Regions Emission nebulae are made of hot, thin gas, which exhibits distinct emission lines:

11.3 Dark Dust Clouds Average temperature of dark dust clouds is 100 K or less These clouds absorb visible light (left), and emit radio wavelengths (right)

11.3 Dark Dust Clouds This cloud is very dark, and can be seen only by its obscuration of the background stars. The image at right is the same cloud, but in the infrared.

11.3 Dark Dust Clouds The Horsehead Nebula is a particularly distinctive dust cloud.

11.3 Dark Dust Clouds The Horsehead Nebula can be seen because it is illuminated by light from behind

11.3 Dark Dust Clouds But what if there isn’t enough light illuminating a dark dust cloud?

11.3 Dark Dust Clouds Interstellar gas emits low-energy radiation, due to a transition in the hydrogen atom:

11.3 Dark Dust Clouds Interstellar gas emits low-energy radiation, due to a transition in the hydrogen atom: Because the wavelength of this 21-cm radiation is much larger than the size of typical dust particles, it can penetrate dust clouds

11.3 Dark Dust Clouds This is a contour map of H2CO near the M20 nebula. Other molecules that can be useful for mapping out these clouds are carbon dioxide and water. Here, the red and green lines correspond to different rotational transitions.

11.3 Dark Dust Clouds These are carbon monoxide-emitting clouds in the outer Milky Way, probably corresponding to regions of star formation.

11.4 Formation of Stars Like the Sun Star formation happens when part of a dust cloud begins to contract under its own gravitational force; as it collapses, the center becomes hotter and hotter until nuclear fusion begins in the core.

11.4 Formation of Stars Like the Sun When looking at just a few atoms, the gravitational force is nowhere near strong enough to overcome the random thermal motion

11.4 Formation of Stars Like the Sun When looking at just a few atoms, the gravitational force is nowhere near strong enough to overcome the random thermal motion But put together about 1057 of them, and you can stop the separation, and start the collapse

11.4 Formation of Stars Like the Sun Stars go through a number of stages in the process of forming from an interstellar cloud:

11.4 Formation of Stars Like the Sun Stage 1: Interstellar cloud starts to contract, probably triggered by shock or pressure wave from nearby star. As it contracts, the cloud fragments into smaller pieces.

11.4 Formation of Stars Like the Sun Stage 2: Individual cloud fragments begin to collapse. Once the density is high enough, there is no further fragmentation. Stage 3: The interior of the fragment has begun heating, and is about 10,000 K.

11.4 Formation of Stars Like the Sun The Orion Nebula is thought to contain interstellar clouds in the process of condensing, as well as protostars.

11.4 Formation of Stars Like the Sun Stage 4: The core of the cloud is now a protostar, and makes its first appearance on the H-R diagram:

11.4 Formation of Stars Like the Sun Planetary formation has begun, but the protostar is still not in equilibrium – all heating comes from the gravitational collapse.

11.4 Formation of Stars Like the Sun The last stages can be followed on the H-R diagram: The protostar’s luminosity decreases even as its temperature rises because it is becoming more compact.

11.4 Formation of Stars Like the Sun At stage 6, the core reaches 10 million K, and nuclear fusion begins. The protostar has become a star. The star continues to contract and increase in temperature, until it is in equilibrium. This is stage 7: the star has reached the Main Sequence and will remain there as long as it has hydrogen to fuse in its core.

11.4 Formation of Stars Like the Sun These jets are being emitted as material condenses onto a protostar.

11.4 Formation of Stars Like the Sun These protostars are in Orion.

11.5 Stars of Other Masses This H-R diagram shows the evolution of stars somewhat more and somewhat less massive than the Sun. The shape of the paths is similar, but they wind up in different places on the Main Sequence.

11.5 Stars of Other Masses If the mass of the original nebular fragment is too small, nuclear fusion will never begin. These “failed stars” are called brown dwarfs.

11.6 Star Clusters Because a single interstellar cloud can produce many stars of the same age and composition, star clusters are an excellent way to study the effect of mass on stellar evolution.

11.6 Star Clusters

11.6 Star Clusters Because a single interstellar cloud can produce many stars of the same age and composition, star clusters are an excellent way to study the effect of mass on stellar evolution.

11.6 Star Clusters This is a young star cluster called the Pleiades. The H-R diagram of its stars is on the right. This is an example of an open cluster.

11.6 Star Clusters This is a globular cluster – note the absence of massive Main Sequence stars, and the heavily populated Red Giant region.

11.6 Star Clusters These images are believed to show a star cluster in the process of formation within the Orion nebula.

11.6 Star Clusters The presence of massive, short-lived O and B stars can profoundly affect their star cluster, as they can blow away dust and gas before it has time to collapse. This is a simulation of such a cluster:

Summary of Chapter 11 Interstellar medium is made of gas and dust Emission nebulae are hot, glowing gas associated with the formation of large stars Dark dust clouds, especially molecular clouds, are very cold. They may seed the beginnings of star formation. Dark clouds can be studied using the 21-cm emission line of molecular hydrogen. Star formation begins with fragmenting, collapsing cloud of dust and gas

Summary of Chapter 11 The cloud fragment collapses due to its own gravity, and its temperature and luminosity increase. When the core is sufficiently hot, fusion begins. Collapsing cloud fragments and protostars have been observed. Mass determines where a star falls on the main sequence. One cloud typically forms many stars, as a star cluster.