Stellar Evolution We have lots of information about stars, but we still need to consider two more areas before we begin to put this all together and see if we can see some kind of “stellar life cycle” (also called stellar evolution). Those last two areas are interstellar material: atoms, dust, and nebula; and variable stars.

Stellar Evolution The Crab Nebula, M1, as imaged by Hubble Space Telescope and the Mount Palomar telescope.

How do we know what is in interstellar space? Gas and dust in space can: scatter light absorb light, heat up, and then re-emit light

Scattered Light In scattering light, blue light scatters more than red light. This gas and dust will then tend to “redden” starlight that passes through it. This effect is seen on the earth – the sky is blue because the blue light is scattered more than the red light; but the sunrise and sunsets appear red because most of the blue has been scattered out of the direct sunlight.

Absorb Light Atoms will selectively absorb light of particular frequencies – called an absorption spectrum. They will later re-emit that light, but in different directions – the emission spectrum. Dust particles will absorb light of most any frequency and tend to heat up. They will emit “blackbody” radiation based on their temperature.

Nebula This combination of absorption and emission of light by gas and dust results in different types of nebula (areas of relatively high gas and dust): dark nebula and glowing nebula. See web sites: Image of the youngest known planetary nebula, the Stingray nebula (Hen-1357).

Interstellar Space Liquid water has about 3 x water molecules per cubic centimeter (in English about 30 billion trillion). Most solids and liquids have similar numbers. At the earth’s surface our atmosphere has about 2.4 x molecules per cubic centimeter (about a thousand times less dense than liquid water). In most of interstellar space, there is about 1 hydrogen atom per cubic centimeter. There are regions of interstellar space, though, that have much higher densities. In nebula, that number can reach a million atoms per cubic centimeter.

Variable Stars While most stars appear to be quite stable, at least on a human time frame, some stars do show variations in brightness. A few show huge changes that appear to be catastrophic events. Others have brightness changes in a very periodic manner, some on the order of seconds, others on the order of days.

Cepheid Variables One type of star, a Cepheid Variable, has a brightness that varies by up to about half a magnitude with a period that ranges from 1 to 100 days. By looking at star clusters where all of its stars appear to be about the same distance away, we find that the period of the star is related to the luminosity of the star! The lower the period, the lower the average luminosity. Cepheid variables with a period of 1 day have an absolute magnitude (luminosity) of about –2. On the other end, Cepheid variables with a period of 100 days have an absolute magnitude of about –8.

Cepheid Variables These are all very luminous stars (giants), and can be seen from very far away. What makes this important are the following relations. We can always measure brightness. It is also easy to measure the period of a Cepheid Variable. With the period-luminosity relationship, we can then get the luminosity. Finally, knowing the brightness and luminosity, we can calculate the distance! This distance determination will be an important tool in Part 5 of the course where we look at the overall size and structure of the universe.

Stellar Evolution Let’s now try to put all of this info together into a theory that will explain our observations and lead us to make further observations to support, refine, or refute our theory.

1. Beginning: Gravitational formation Most of space is fairly empty of matter and cold. However, there are areas of relatively high gas and dust (nebula). Over time, gravity will tend to pull the gas and dust together. As it does, it will tend to convert the gravitational energy into heat energy (the speed of falling is converted into heat).

1. Beginning: Gravitational formation The “cloud” of gas and dust will tend to get smaller and hotter. A smaller size tends to reduce the luminosity, but hotter tends to increase luminosity. The position of the newly forming star on the H-R diagram will move to the left as it heats up but wander up and down somewhat as its size shrinks. This process takes about 50 million years for a star like the sun, but may take a much shorter time for a more massive star since there will be more gravity. A ten solar mass star will only spend about 200,000 years in this initial stage.

H-R Diagram Gravitational formation LuminosityLuminosity Temperature / Color O0B0A0F0G0K0M0 Sun = G2 at +4.8 Magnitude 1

2.Nuclear Fusion of Hydrogen Stability on the Main Sequence When the temperature and pressure at the core of the newly forming star reaches a certain point, the hydrogen atoms will collide with one another so hard that nuclear fusion will occur (basically four hydrogen atoms combine to form one helium atom plus LOTS of energy). This “hydrogen bomb” process tends to blow the star apart, but gravity continues to try to collapse the star.

2. Nuclear Fusion of Hydrogen Stability on the Main Sequence The result of these competing tendencies is a stable star, both in size and in temperature (and hence in position on the H-R diagram on the Main Sequence). More massive stars have more fuel, but they also have more gravity that causes the core to burn the fuel at a faster rate than less massive stars. The result is that more massive stars are hotter and more luminous and are higher on the Main Sequence than less massive stars, and they remain stable on the Main Sequence for less time.

2. Nuclear Fusion of Hydrogen Stability on the Main Sequence A star like the sun will last about 10 billion years on the Main Sequence. A star with 15 times the mass of the sun will only last about 10 million years on the Main Sequence. In the same way, stars with less mass then the sun will stay on the main sequence much longer than 10 billion years.

3.Red Giant Nuclear Fusion of Helium When the hydrogen starts to run out in the core, the explosive energy production of nuclear fusion no longer can balance the gravitational tendency to collapse, and so the core of the star will again start to collapse while hydrogen is still burning on the outside of the core. This gravity collapse of the core will again heat up the core, and this extra heat will cause the star’s surface to expand. As the surface expands, it will tend to cool. The result is a red giant state – higher luminosity but a little cooler surface.

3. Red Giant Nuclear Fusion of Helium For a star like the sun, this expansion of the surface will be large enough to reach the orbit of Venus or even the Earth. When the core gets hot enough, it will start to have the helium atoms (ashes of the hydrogen fusion) combine in nuclear fusion to form carbon and release energy. This process takes roughly about 10% of the time of the Main Sequence hydrogen burning.

H-R Diagram Red Giant LuminosityLuminosity Temperature / Color O0B0A0F0G0K0M0 Sun = G2 at +4.8 Magnitude 1 2 3

4. Unstable stars After the helium fuel in the core runs out, there are different scenarios for different masses of stars. For a star with about the mass of the sun or less, the core will again collapse and the gravitational energy of the collapse will eject some of the outer layers of the star (called planetary nebula ejection) and the core (now at about 0.6 of the original mass of the star) will heat up (move to the left and tend to move up) and shrink (tend to move down).

H-R Diagram Unstable LuminosityLuminosity Temperature / Color O0B0A0F0G0K0M0 Sun = G2 at +4.8 Magnitude eject planetary nebula Cepheid Variables

4. Unstable stars For more massive stars, the situation is more complicated. With the higher gravity, the core can get hot enough to start burning the carbon to get even heavier elements. This proceeds until the core turns into iron. Since the nucleus of iron is tightest bound of all atoms, iron cannot undergo nuclear fusion to release energy like the less massive atoms can.

4. Unstable stars When the core cannot continue with fusion, there is nothing to balance gravity, and the core will totally collapse. The implosion of the core will release so much energy that it will blow the outer parts of the star completely away in a supernova explosion.

Final Stage – Death of the Star There are three possibilities for the collapsed core depending on the mass of the remaining core: 1) If the final mass after the planetary nebula release is less than 1.4 solar masses, the remaining mass of the star will collapse down to a size about that of the earth. It will be a white dwarf star, and then as it cools it will become a brown dwarf and then eventually cool even further.

H-R Diagram Final Stage: Death LuminosityLuminosity Temperature / Color O0B0A0F0G0K0M0 Sun = G2 at +4.8 Magnitude eject planetary nebula Cepheid Variables White dwarf

Final Stage – Death of the Star 2)If the final mass (after the supernova explosion) is more than 1.4 solar masses but less than about 3 solar masses, the core will stop collapsing when the atoms are so compacted that the electrons are shoved into the protons and the whole mass becomes neutrons that stick together by gravity. This is called a neutron star. It’s diameter is only about 20 kilometers (compared to about 12,000 kilometers for a white dwarf!).

Pulsar 2-continued) If the original star had an appreciable magnetic field and a rotation, the resulting neutron star may still have that magnetic field and it will have a much higher rotational speed due to the collapse. The magnetic field may cause light to be emitted in a beam, and with the rotation this beam may rotate at a high angular speed. We have seen pulses of light with periods of a few seconds from these spinning neutron stars and so we call them pulsars.

Final Stage – Death of the Star 3) If the final mass of the core after the supernova explosion is more than about 3 solar masses, then gravity is so strong it will collapse the matter even beyond the neutron star size. We know of nothing that would stop the collapse. This is called a black hole.

Black Holes Note that the mass of a black hole is still there and its gravity will affect things around it. But gravity is so strong near it that even light can be trapped so that it does not escape from the black hole. Further away, though, other stars will feel the gravity just like they feel the gravity of other massive objects.

Mass back to nebula and space In the ejection of the planetary nebula and in supernova explosions, some and sometimes most of the mass of the star is ejected back into space. There is a difference, though. The initial mass of the collapsing nebula consisted of mostly hydrogen. The final mass of the expanding nebula is enriched in the heavier elements. The energy in a supernova is so high that elements heavier than iron are made.