The Interstellar Medium

LEARNING OBJECTIVES

By the end of this section, you will be able to:

• Explain how much interstellar matter there is in the Milky Way, and what its typical density is
• Describe how the interstellar medium is divided into gaseous and solid components

Astronomers refer to all the material between stars as interstellar matter; the entire collection of interstellar matter is called the interstellar medium (ISM). Some interstellar material is concentrated into giant clouds, each of which is known as a nebula (plural “nebulae,” Latin for “clouds”). The best-known nebulae are the ones that we can see glowing or reflecting visible light; there are many pictures of these in this chapter.

Interstellar clouds do not last for the lifetime of the universe. Instead, they are like clouds on Earth, constantly shifting, merging with each other, growing, or dispersing. Some become dense and massive enough to collapse under their own gravity, forming new stars. When stars die, they, in turn, eject some of their material into interstellar space. This material can then form new clouds and begin the cycle over again.

About 99% of the material between the stars is in the form of a gas—that is, it consists of individual atoms or molecules. The most abundant elements in this gas are hydrogen and helium (which we saw are also the most abundant elements in the stars), but the gas also includes other elements. Some of the gas is in the form of molecules—combinations of atoms. The remaining 1% of the interstellar material is solid—frozen particles consisting of many atoms and molecules that are called interstellar grains or interstellar dust (Figure 20.2). A typical dust grain consists of a core of rocklike material (silicates) or graphite surrounded by a mantle of ices; water, methane, and ammonia are probably the most abundant ices.

Figure 20.2 Various Types of Interstellar Matter. The reddish nebulae in this spectacular photograph glow with light emitted by hydrogen atoms. The darkest areas are clouds of dust that block the light from stars behind them. The upper part of the picture is filled with the bluish glow of light reflected from hot stars embedded in the outskirts of a huge, cool cloud of dust and gas. The cool supergiant star Antares can be seen as a big, reddish patch in the lower-left part of the picture. The star is shedding some of its outer atmosphere and is surrounded by a cloud of its own making that reflects the red light of the star. The red nebula in the middle right partially surrounds the star Sigma Scorpii. (To the right of Antares, you can see M4, a much more distant cluster of extremely old stars.) (credit: modification of work by ESO/Digitized Sky Survey 2)

If all the interstellar gas within the Galaxy were spread out smoothly, there would be only about one atom of gas per cm³ in interstellar space. (In contrast, the air in the room where you are reading this book has roughly 10¹⁹ atoms per cm³.) The dust grains are even scarcer. A km³ of space would contain only a few hundred to a few thousand tiny grains, each typically less than one ten-thousandth of a millimeter in diameter. These numbers are just averages, however, because the gas and dust are distributed in a patchy and irregular way, much as water vapor in Earth’s atmosphere is often concentrated into clouds.

In some interstellar clouds, the density of gas and dust may exceed the average by as much as a thousand times or more, but even this density is more nearly a vacuum than any we can make on Earth. To show what we mean, let’s imagine a vertical tube of air reaching from the ground to the top of Earth’s atmosphere with a cross-section of 1 square meter. Now let us extend the same-size tube from the top of the atmosphere all the way to the edge of the observable universe—over 10 billion light-years away. Long though it is, the second tube would still contain fewer atoms than the one in our planet’s atmosphere.

While the density of interstellar matter is very low, the volume of space in which such matter is found is huge, and so its total mass is substantial. To see why, we must bear in mind that stars occupy only a tiny fraction of the volume of the Milky Way Galaxy. For example, it takes light only about four seconds to travel a distance equal to the diameter of the Sun, but more than four years to travel from the Sun to the nearest star. Even though the spaces among the stars are sparsely populated, there’s a lot of space out there!

Astronomers estimate that the total mass of gas and dust in the Milky Way Galaxy is equal to about 15% of the mass contained in stars. This means that the mass of the interstellar matter in our Galaxy amounts to about 10 billion times the mass of the Sun. There is plenty of raw material in the Galaxy to make generations of new stars and planets (and perhaps even astronomy students).

EXAMPLE 20.1

Estimating Interstellar Mass

You can make a rough estimate of how much interstellar mass our Galaxy contains and also how many new stars could be made from this interstellar matter. All you need to know is how big the Galaxy is and the average density using this formula:

total mass=volume×density of atoms×mass per atomtotal mass=volume×density of atoms×mass per atom

You have to remember to use consistent units—such as meters and kilograms. We will assume that our Galaxy is shaped like a cylinder; the volume of a cylinder equals the area of its base times its height

V=πR2h𝑉=π𝑅2ℎ

where R𝑅 is the radius of the cylinder and hℎ is its height.

Suppose that the average density of hydrogen gas in our Galaxy is one atom per cm³. Each hydrogen atom has a mass of 1.7 × 10⁻²⁷ kg. If the Galaxy is a cylinder with a diameter of 100,000 light-years and a height of 300 light-years, what is the mass of this gas? How many solar-mass stars (2.0 × 10³⁰ kg) could be produced from this mass of gas if it were all turned into stars?

Solution

If the diameter of the Galaxy is 100,000 light-years, then the radius is 50,000 light-years. Recall that 1 light-year = 9.5 × 10¹² km = 9.5 × 10¹⁷ cm, so the volume of the Galaxy is

V=πR2h=π(50,000×9.5×1017cm)2(300×9.5×1017cm)=2.0×1066cm3𝑉=π𝑅2ℎ=π(50,000×9.5×1017cm)2(300×9.5×1017cm)=2.0×1066cm3

The total mass is therefore

M=V×density of atoms×mass per atom𝑀=𝑉×density of atoms×mass per atom

2.0×1066cm3×(1atom/cm3)×1.7×10–27kg=3.5×1039kg2.0×1066cm3×(1atom/cm3)×1.7×10–27kg=3.5×1039kg

This is sufficient to make

N=M(2.0×1030kg)=1.75×109𝑁=𝑀(2.0×1030kg)=1.75×109

stars equal in mass to the Sun. That’s roughly 2 billion stars.

Check Your Learning

You can use the same method to estimate the mass of interstellar gas around the Sun. The distance from the Sun to the nearest other star, Proxima Centauri, is 4.23 light-years. We will see in Interstellar Matter around the Sun that the gas in the immediate vicinity of the Sun is less dense than average, about 0.1 atoms per cm³. What is the total mass of interstellar hydrogen in a sphere centered on the Sun and extending out to Proxima Centauri? How does this compare to the mass of the Sun? It is helpful to remember that the volume of a sphere is related to its radius:

V=(4/3)πR3𝑉=(4/3)π𝑅3

ANSWER

The volume of a sphere stretching from the Sun to Proxima Centauri is:
V=(4/3)πR3=(4/3)π(4.2×9.5×1017cm)3=2.7×1056cm3𝑉=(4/3)π𝑅3=(4/3)π(4.2×9.5×1017cm)3=2.7×1056cm3
Therefore, the mass of hydrogen in this sphere is:
M=V×(0.1atom/cm3)×1.7×10–27kg=4.5×1028kg𝑀=𝑉×(0.1atom/cm3)×1.7×10–27kg=4.5×1028kg
This is only (4.5 × 10²⁸ kg)/(2.0 × 10³⁰ kg) = 2.2% the mass of the Sun.