### Star light, star bright

This exercise is a part of Educator Guide: Neutron Star Crash Seen for First Time / View Guide

Directions: After students have had a chance to review the article “Neutron star crash seen for first time,” lead a classroom discussion based on the questions that follow.

#### Discussion questions:

1. How can different wavelengths of light be separated to study the spectrum (spectroscopy) of an object in space?

When electromagnetic radiation passes from air (or the vacuum of space) into a prism — including ones found in telescopes — the higher density of the prism causes the light waves to slow and bend. The refraction separates the various wavelengths of light, which causes the separated light waves to leave the prism at slightly different angles. Similarly, light of different wavelengths passing through a diffraction grating is bent by varying degrees depending on the wavelength. This is called dispersion due to diffraction, and happens because the different wavelengths of light interact differently with vertical bars in diffraction grating, which have a fixed spacing comparable to some wavelengths of light. Both refraction and diffraction are used in spectroscopy.

2. How can spectroscopy be used to determine the chemical composition of stars, colliding neutron stars, nebulae (gas clouds) or other objects in space?

Spectroscopy measures electromagnetic radiation (including infrared, ultraviolet, visible light, x-rays and gamma rays) that is emitted, absorbed or scattered by various materials. All elements absorb and emit radiation at specific energies, which result in radiation patterns unique to each element. Those patterns act as a sort of fingerprint, and can help scientists identify and quantify the chemical composition of objects in space.

Guide to Spectroscopy and Spectral Lines
Astronomy Tools: Spectroscopy
Examples of Spectroscopy in Astronomy

#### Extension prompts:

3. The curve of binding energy shows how tightly bound the nucleons (protons and neutrons) are within a nucleus, depending on the mass of the nucleus. Based on a binding energy curve, similar to the one titled “Fission and fusion can yield energy” found on the HyperPhysics page hosted by Georgia State University, what element appears to have the most stable nucleus? How do you know from the graph?

Iron has the most stable nucleus. All isotopes of iron have the most tightly bound nucleons, meaning they have the highest binding energy per nuclear particle (as this graph shows, the binding energy per nucleon is around 8.8 million electron volts (MeV) for nuclei with masses of about 55 to 60 atomic mass units). It is important to note that nickel-62 actually has the most stable nucleus, but it is not as abundant in stellar cores as iron-56, so astrophysicists use iron to focus their reasoning and explanations.

4. Fusion reactions join small nuclei together to create larger nuclei. Based on that graph, would fusion reactions that produce nuclei that are less massive than iron consume or release net energy?

Fusion reactions that produce nuclei up to iron’s atomic mass can release net energy. The protons and neutrons in the initial smaller nuclei are more weakly bound, but they become more strongly bound in the larger nuclei. That difference in their binding energy is released as net energy in the reaction — the reaction is exothermic.

5. Based on that graph, would fusion reactions that result in nuclei that are more massive than iron consume or release net energy?

Fusion reactions can also produce nuclei larger than iron. But those large nuclei have more weakly bound protons and neutrons, so producing such nuclei consumes a great deal of energy — the reaction is endothermic. The energy demands are why heavy elements are mainly only created in high-energy events, such as the collision of neutron stars or during supernovas.

6. For a given mass of fuel, which releases more energy — fusion of very light elements to form a stable nuclei, or fission of a very heavy element to form stable nuclei? Use the graph.

Fission of a heavy element such as 235U all the way to the most stable element (iron-56) would release roughly 1 MeV per nucleon. Fusion of very light elements, such as hydrogen, all the way to the most stable element (iron-56) would release over 8 MeV per nucleon. Thus, fusion reactions produce much more energy than fission for a given mass of fuel.

#### Discussion questions:

1. How does a star form?

A protostar is a cloud of mostly hydrogen that contracts and heats up due to gravitational forces to form a star. Fusion reactions called proton-proton reactions, which occur during fusion, become important once the central temperature rises to about Tc = 8 x 106 K. If the protostar has a sufficiently large mass, it will contract until the central temperature is hot enough to produce fusion reactions, and a star will be born.

2. How does a star live?

Stars rely on fusion to survive. In one type of fusion reaction, the heat and pressure at the center of a star are so great that hydrogen reacts to become helium. In a series of proton-proton, or pp, reactions, four protons are progressively joined together to form helium-4 nuclei called alpha particles. Along the way, two of the protons undergo inverse beta decay to become neutrons. Energy from those fusion reactions maintain the heat and pressure in the core, pushing outward and counterbalancing the inward pull of gravity.

Because both the temperature and the density decrease with radius, almost all of the fusion occurs in the core of the star (provided that unburned fuel remains there).

3. How does a star die?

Toward the end of its life, after a star has consumed most of the hydrogen in its core, the core collapses and heats up the rest of the star until it is hot enough to fuse the hydrogen that remains in the star’s mantle; this is called shell burning. The fusion reactions in the mantle cause the surface layers of the star to expand and cool, resulting in an enormous red giant star. Further contraction and heating of the core can lead to the fusion of helium and progressively heavier elements.

In these final stages of fuel consumption, a star often sheds its outer layers. The remaining stellar core collapses to form one of three objects, depending on the star’s mass:

a.  White dwarf. For stars with masses comparable to our sun or smaller stars, the star contracts until the only thing keeping it from collapsing into a black hole is the fact that its electrons cannot share a quantum state, which is known as electron degeneracy pressure. That pressure stabilizes it, forming a white dwarf that slowly radiates away its residual energy.

b.  Neutron star. For stars with masses somewhat larger than our sun, gravity overcomes the degenerate electron pressure at the white dwarf stage. As the star goes supernova (explodes), its core continues to contract until it squeezes its protons and electrons together to form neutrons. The resulting neutron star acts like a giant nucleus of neutrons and is stopped from further collapse by the neutron degeneracy pressure. Eventually the neutrons become so close that they would need to share a quantum state to get any closer, too.

c.  Black hole. If the mass of a star is several times larger than that of our sun, gravity overcomes the neutron degeneracy pressure at the neutron star stage and the star collapses, becoming a black hole. Nothing — not even light — that ventures close enough can resist being sucked in by the extreme gravitational field.

#### Extension prompts:

4. What is a Hertzsprung-Russell diagram?

Hertzsprung-Russell diagrams plot how much light a star produces, called stellar luminosity, versus the star’s surface temperature (related to a star’s color — redder is cooler and bluer is hotter). Most stars in the universe, including Earth’s sun, spend a majority of their life spans on the main sequence, a distinct band of stars on the Hertzsprung-Russell diagram. After consuming most of their fusion fuel, these stars move off the main sequence. What happens to them next depends on their initial masses: stars up to 10 solar masses become red giants, while stars below 0.2 solar masses become white dwarfs. Eventually, all stars collapse when all of the fuel is exhausted.

#### Discussion questions:

1. How long did it take light and gravitational waves to travel from the neutron star collision to Earth? How long ago did the collision actually happen?

Scientists determined that the neutron star collision occurred in the galaxy NGC 4993, 130 million light-years from Earth in the constellation Hydra. A light-year is the distance that light travels in a year, so the collision must have occurred approximately 130 million years ago if we are just now seeing light produced from it.

2. The New Horizons probe that flew by Pluto in 2015 is traveling away from our solar system at approximately 52,000 kilometers per hour. What fraction of light speed is that? If New Horizons were traveling toward NGC 4993 and kept that same velocity, how long would it take to reach from Earth whatever is left of the neutron star collision?

New Horizons is traveling at approximately 1/20,800 or 0.0000481 times the speed of light. It would take the probe about 2.7 trillion years to reach the approximate location of the recently detected neutron star collision.

#### Extension prompts:

3. What sorts of things could we monitor or learn from satellites with similar sensors that are pointed inward toward Earth instead of outward toward space?

Satellites that detect infrared light could tell us information about water, land and air temperatures as well as fires and explosions. Satellites that detect X-rays and gamma rays could tell us information about above-ground nuclear tests. Satellites that detect radio waves could monitor communications.

4. How might the gravitational wave detectors be improved?

If the interferometers had longer arms, the detectors could be more sensitive. If there were more detectors watching at the same time, it would be easier to triangulate the position of the gravitational waves’ source in the sky. If some of the detectors were in space, they could help to better eliminate sources of Earth-based noise from the signals. Space-based detectors could also measure gravitational waves from different sources, like colliding supermassive black holes, which emit gravitational waves at longer wavelengths.

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