Astronomy 101

1. Astronomy 101 Lecture 19, Apr. 2, 2003 Neutron Stars and Gamma Ray Bursters– (Chapter 22.1 – 22.4 in text) In last lecture, we saw a Type II supernova blowing off its outer layers in a gigantic explosion, due to a shock wave that bounced off the central neutron core. The supernova lights up the sky and seeds the neighborhood with heavy elements used for complex life. What’s left of the star after the explosion? If the mass of the sphere of neutrons is less than about 3 solar masses, it remains as a Neutron star. Recall that the neutron core formed just before the supernova erupted had very high density. The neutrons are as close as quantum mechanics will allow them (degenerate). The density of the neutron star is about the same as inside an nucleus of the atom – but for the neutron star, instead of having 10 – 200 neutrons and protons, there are now about 1057 of them inside the sphere. And there are no protons left – all were converted to neutrons by the reaction p + e-→ n + n just preceding the final collapse

2. The density inside the neutron star is huge – about 1018 kg/m3, or a million billion times the density of water. So the size of the neutron star is tiny by star standards – about 20 kilometers across. The large mass and small radius means that gravity is incredibly strong at its surface: Fgrav = GMm/R2 A 150 pound person would weigh about a million tons on the surface. There is no way to burn neutrons into anything else, so no new heat is generated. The star cannot contract further due to the degenerate neutron pressure. So the neutron star just radiates blackbody radiation and cools off. neutron star Manhattan

3. The neutron star rotates very rapidly, because of the conservation of angular momentum. For any object, the angular momentum – the product of its mass times rotational velocity times radius – is conserved. The star before collapse had a radius of about 1000 solar radii and turned on its axis perhaps once per month. The neutron star is only 10 km in radius, and turns on its axis many times per second to maintain the same angular momentum. In addition, the neutron star has very strong magnetic fields. These too get larger during the collapse as they are due to electric currents that become larger as the star becomes smaller and rotates faster. In time, the rotation rate and the magnetic fields become smaller due to the radiation of energy from the neutron star.

4. Observing a neutron star We see spinning neutron stars as PULSARS. In 1967 a graduate student at Cambridge saw a light curve from a star (intensity vs. time) with very short pulses every 1.34 seconds. 1.34 seconds The pulse period is extremely regular, and initially people speculated that they came from LGMs (little green men) as evidence of a civilized society beaming us signals from deep space. By now we have seen hundreds of these pulsars, each with its own distinct pattern and period. Some are seen in visible light, others in X-rays.

5. How does the neutron star pulsar work? Magnetic field lines emerge from the magnetic south pole, circle around and come back into the neutron star at the north magnetic pole. The magnetic field is particularly strong near the poles. The interstellar gas of ions and electrons circle the magnetic field and fall toward the poles. When they bend in the field, they emit ‘synchrotron radiation’ in the form of radio waves, light, ultraviolet, and X-rays. Most of this radiation is emitted along the magnetic pole axes. The magnetic field lines look approximately like the pattern of iron filings surrounding a short bar magnet. rotation axis S pole magnetic axis N pole

6. The magnetic pole axis is not typically aligned with the rotation axis. So the beams of light along the magnetic axes turns in space like a searchlight. If the earth is in the line of the searchlight, we see the pulses every time the star rotates. Magnetic axis neutron star If earth is here, we don’t see the pulses. radiation Axis of rotation If earth is here, we do see the pulses.

7. Geminga pulsar seen in images taken 0.05 seconds apart – the (gamma ray) intensity blinks on and off with a 0.24 sec. Period. Nearest neutron star to us (at 60 pc) is not a pulsar. It was discovered by Stony Brook astronomers. It is moving across the sky at 110 km/s, faster than typical for stars. The high speed is probably due to a kick given during the supernova explosion. This neutron star is seen as an X-ray source.

8. If the neutron star is in a binary pair, matter (hydrogen) can be transferred from the normal companion to the neutron star. When enough hydrogen accumulates to raise the temperature, it ignites in the very strong gravitational field and emits a burst of X-rays. This mechanism is very similar to what happens when gas from a companion falls on a white dwarf to form a nova, but more powerful. The infalling gas spirals in and hits the neutron star moving parallel to its surface. This impact can speed up the rotation, leading to pulsars with very short (0.001 sec) periods. Such pulsars have surfaces moving at a quarter the speed of light!

9. Gamma ray bursters Military satellites looking for bomb blasts on earth found bursts of gamma rays in the universe, lasting from a fraction to several seconds. They occur uniformly over the sky, so seem to be outside our galaxy (which lies in a single plane).

10. Some gamma ray bursters have been identified with optical objects that are moving away from us with huge speeds, using the observed Doppler shifts of known spectral lines. This indicates that they are very very far away (we will make this connection between recessional velocity and distance clear later – it’s called the Hubble expansion of the universe). From the energy we observe on earth and knowing the distance, we can calculate the total luminosity of the gamma ray burster. Assuming that the burster radiates equally in all directions, we find that they are hundreds of times more luminous than a supernova – the brightest known objects in the sky. We know they are small, since there are substantial changes in intensity in milliseconds – only possible for small objects. Stony Brook scientists propose that Gamma Ray bursters arise from a rotating black hole whose powerful magnetic field sweeps through the accretion disk formed from a companion star. There is not yet consensus on what makes the GRBs work!

11. Midterm Exam 2 – Monday April 7, in class. Bring your ID, pencil for marking the Scantron form. Be sure to record the exam type in the first digit labelled ‘birth date’ (exam types 0, 1, 2, 3) Exam will be of similar form to first midterm – 40 multiple choice questions and two essay questions or problems. As review, lets look at AST101 Midterm 2 in Fall 2002.

12. In the proton-proton chain, an important intermediate step involves the formation of 2H (deuteron). 2H is: • A hydrogen atom with 2 electrons • A hydrogen nucleus with 2 protons • A hydrogen nucleus with 2 nucleons: 1 proton and 1 neutron • A hydrogen nucleus with 2 neutrons • An isotope of helium • A cubic centimeter of an interstellar molecular cloud contains about _____ molecules of hydrogen on average. • 3 • 30 • 300 • 300,000 • 3,000,000

13. A star with parallax of 0.2 arc seconds is at a distance of about • 5 AU • 500 AU • 0.5 parsecs • 5 parsecs • 0.2 parsecs d (pc) = 1/parallax angle (arcsec) • If the mass of the sun was 10 times its value and Earth remained in an orbit at 1 AU (150 million km), the length of an Earth year would be: • About the same as now • About 10 times shorter than now • About 10 times longer than now • About 3 times longer than now • About 3 times shorter than now Newton’s modification of Kepler’s 3rd Law P2 = a3 / (Msun + Mearth) P2 reduced by about factor 10, so P reduced by √10 ~ 3.

14. The mass of a giant molecular cloud is: • About the mass of the earth • About the mass of the sun • About the mass of 100 suns • About the mass of 100,000 suns • The effect of interstellar dust on starlight is: • To make stars appear less bright than expected, by absorbing light about equally at all wavelengths • To scatter the red light from stars preferentially, making them appear more blue than expected • Almost nothing, since light does not interact with dust • To dim and redden distant stars by preferentially scattering their blue light.

15. Emission light UV • Where in the universe would you look for a protostar? • In giant molecular clouds • In globular clusters of stars • In the empty space between the galaxies • Near to black holes • In supernova remnants The power source for the light from an emission nebula (H II region) is: • Ionization of hydrogen by ultraviolet light from hot stars • Ionization of hydrogen by infrared light from hot stars • An electric current running from the nebula from a hot star • Reactions between hydrogen and oxygen which supply explosive energy • None of the above

16. In the interstellar medium, radio waves of 21 cm wavelength originate in which component? • Ionized atomic hydrogen • Carbon monoxide, CO • Molecular hydrogen • Neutral atomic hydrogen Relative orientation of proton and electron spins changes from parallel to antiparallel and emits 21 cm radiation. • A standard candle is a star or object for which we know the __________, and measure _________________ to determine the ___________ . • Size, apparent brightness, distance • Luminosity, apparent brightness, distance • Distance, luminosity, apparent brightness • Luminosity, distance, apparent brightness • Apparent brightness, luminosity, distance luminosity Apparent brightness distance L = 4pd2 Iapp

17. What important stellar parameter can be measured from observations of binary stars, but not single stars? • Surface temperature • Age of star • Distance of star from earth • Stellar mass • Both C and D • The key property that distinguishes stars on the main sequence from all others is: • They have the same mass • They are burning hydrogen • They are in gravitational equilibrium • They are burning hydrogen in their centers • They are the same age

18. Which of the following is true about the rate of stellar evolution? • The more massive the initial star, the slower the evolution since more material for nuclear burning • The more massive the initial star,the faster the evolution • Star mass has no bearing on evolution since all stars evolve at same rate • The chemical makeup of the original nebula is the major factor in determining the rate of evolution, whatever the mass. • How long will the sun have spent as a main sequence star when it finally becomes a red giant? • 1 billion years • 1 million years • 1011 years • 1010 years • 1012 years 1010 = 10 billion

19. Our sun will ultimately become: • Neutron star • White dwarf • Pulsar • Black hole • Supernova • Thermonuclear burning and fusion can produce such ‘heavy’ elements as carbon, oxygen etc. up to a limit beyond which no further energy producing reactions can occur. This limit occurs at element: • Uranium • Oxygen • Iron • Silicon • None of the above

20. When a star has exhausted hydrogen at its core • The whole star expands, becoming bigger and less luminous • The core contracts, the surface expands, the surface cools and the luminosity increases. • The core and surface contract, making it hotter and brighter • The core contracts, the surface expands, the surface cools and the luminosity decreases • It contracts, becoming smaller and less luminous. • A white dwarf generates energy from what source? • Gravitational potential energy as star slowly contracts • No longer generates energy, but cools slowly • Nuclear fusion of hydrogen into helium • Nuclear fusion of heavy elements in the central core

21. A star of 20 solar masses will end its life as a: • A planetary nebula, leaving a neutron star • As a supernova, leaving a white dwarf • As a supernova,leaving a neutron star or black hole • As a planetary nebula, leaving a white dwarf • As a supernova, leaving no remnant • Which major astronomical event was recorded by Chinese astronomers in 1054 AD? • A supernova explosion in our galaxy, visible even in daytime • A supernova in another galaxy, visible even in daytime • A total eclipse of the sun • A formation of a planet • The birth of a new star

22. A physical characteristic of matter in a white dwarf is • Extemely high density compared to ordinary stellar matter • Composed only of electrons in degenerate state • Composed only of neutrons • Is in the form of a hollow shell with a black hole in the center • Is composed only of iron • Stars which have ejected a planetary nebula go on to become • Protostars • Supernovae • Red giants • White dwarfs

23. A supernova of Type II (from a massive star) is powered by: • Gravitational collapse • Convection • Fusion • Fission • B and C Actually, I’d prefer to say a Type II supernova is powered immediately by an expanding shock wave, triggered by gravitational collapse of material onto the brick wall of neutron degenerate pressure. • Stellar nucleosynthesis refers to • All formation of complex chemicals in interstellar space • The synthesis of stars from interstellar matter • The transfer of energy by synthesis • The formation of chemical elements in the periodic table by nuclear fusion inside stars, starting with hydrogen • None of the above

25. Energy can be transported across empty space by: • Convection • Radiation • Conduction • Convection and radiation • Cannot be transported in empty space without matter • A cubic centimeter of air in this room contains about _____ molecules of nitrogen and oxygen. • 1000 or less • 100,000 • 108 • 1011 • More than 1014

26. At what wavelengths have astronomers mapped and studied the distribution of giant molecular clouds in space? • UV, because molecules are efficient UV emitters and the clouds are hot • Millimeter wavelengths, using radio telescopes • Visible light using photography • Using ultrasound waves at high frequency • Why does hydrogen ignite in a shell around the helium core after the main sequence? • Cooling due to cessation of core fusion ignites the hydrogen • Higher density due to core contraction starts off the fusion • The hydrogen was already burning, it just continues • The higher temperature due to core contraction starts off the fusion • The decrease in light from the core reduces the repulsion of the hydrogen nuclei

27. What is a planetary nebula? • The cloud from which a protostar forms • The leftovers of the supernova explosion that formed a white dwarf in the middle • The disk surrounding a newborn star • The leftovers of the planets after the star has died • The expelled envelope of a low or intermediate mass star at the end of its life.