Goal: To understand special stars. Objectives: 1)To learn about Black holes 2)To learn about Neutron Stars 3)To understand Stars that erupt. 4)To understand Variable stars

Special stars – neutron stars Neutron stars are stars that are about 1.4 times the mass of our sun and made entirely of neutrons. These stars are only a few km in size. They are essentially a giant atom! Densities are HUGE! They also spin and have magnetic fields. Pictured is the Crab Nebula – which supernovaed in 1054.

Special stars – Pulsars Pulsars are neutron stars. They spin very quickly (once per second to a thousand times per second). The stars have strong magnetic fields, and only beam light from their pole (sort of like a lighthouse floodlight). The pulses normally come in the radio. However, they also emit a lot of X-rays. The Crab for example spins 30 times per second.

Energy has to come from somewhere. Where does the energy the pulsars emit come from? A) heat B) nuclear fusion C) gravity D) Spin

Neutron Stars in binary systems Remember that most stars are in binary systems! At the end of the life of the biggest star, sometimes the other stars get away because the dying star looses a lot of mass. Sometimes they stay together. Then, when the smaller star evolves…

binary systems – Roche Lobes As a star expands it has a looser and looser hold on its own materials (gravity decreases by the radius squared). At some point a companion star will have more influence over the outermost parts of the star than the star itself does! This is called the Roche Lobe. If a red giant expands past its Roche Lobe, the companion star will accrete materials from it.

But, what happens when you accrete matter onto a few km ball of mostly neutrons? Well, at first the Hydrogen falls way down onto the surface. This produces energy that helps to power the constant emission of X-rays by the neutron star. Then, the H is fused into He and crushed onto the surface of the neutron star. Soon you build up a layer of He (sort of like a layer of snow).

He bomb When the He layer is about 1 m thick, the Helium ignites! As we saw with the Helium flash for a star, this is a tricky time. The burning He heats the surface of the star – which speeds up the production of Helium! The result is a spectacular explosion (although not as spectacular as a supernova) This produces an X-ray burster!

One other side effect Another side effect of accreting matter is a change to the spin. Will the spin get faster or slower?

White dwarfs in binaries White dwarfs also can flare up in a binary system. On a white dwarf though, the matter falling in stays as Hydrogen. Eventually the temperatures on the surface go up and the density of Hydrogen gets high enough to fuse into Helium. This creates a quick burning which is known as a nova (not to be confused with supernova).

Black Holes in binaries Just like with Neutron stars and white dwarfs, black holes will create an accretion disk. However, you can see nothing from the actual accretion, so all you get to see is the accretion disk. On the plus side, the accretion disk goes down to a few km in size at which point the gas has been heated quite a bit (infalling gas is slowed by frictional heating and interactions with the magnetic field). The innermost parts will emit X-rays!

Astro-mercial But wait there’s more! JETS! Materials racing outward at close to the speed of light and going for up to millions of light Years! (NGC 5532)

Variable stars Usually stars are held in equilibrium. If they expand then they cool and that ends the expansion. Their cores are stable. The star is stable. However, there is a region on the HR diagram where this is not the case – the instability strip. In this region stars will pulsate – that is they will expand and contract. This causes the star to get brighter and dimer.

Variable types RR Lyrae – lower mass stars after they undergo their Helium flash (the sun will do this someday). RR Lyrae are Horizontal Branch stars. Metal rich and Metal poor Cepheid variables (Type I and II). These are the higher mass stars which pass back and forth through the instability strip.

Observing RR Lyrae RR Lyrae have periods of about 0.3 to 0.5 days. A) Why do you think those periods are so short? B) Why is this length of period a really bad thing when it comes to observing the star (hint, when can a good telescope look at stars?)?

Absolute Magnitude RR Lyrae have an average absolute magnitude of Why is that an advantage? What is the disadvantage if you are looking at other galaxies?

Why variable stars are important Variable stars have a relationship between their period of pulsation and their absolute brightness. The longer the period, the bigger the star is, and the brighter it is (sort of like a bigger bell has a larger period of vibration). This allows us to measure distances (especially since these are very bright stars which can be seen a LONG distance away)! In fact, the distance to Andromeda was first attempted to be estimated using Cepheid variables.

Distance to Andromeda Edwin Hubble tried to estimate the distance to Andromeda using Type II Cepheids (metal poor). Type II Cepheids are in the globular clusters. However, he made a slight mistake.

Type I Cepheids (metal rich): Mv = log(Period in days) Type I Cepheids (metal rich) ones in the disk of our galaxy have a pretty exact relationship between variability period and average absolute magnitude. The brightness of Type Is is 4 times greater than Type IIs

Distance misestimated So, Hubble underestimated the distance to Andromeda by a factor of 50.

Even today We still know the distance to the Andromeda galaxy using the Type I (metal rich) Cepheids.

Profile of a Cepheid Variable Cepheids expand and contract. As they do they change color (and temperature). As they expand they cool and turn redder. As they condense they get hotter and turn bluer. When do you think they are brightest?

Conclusion There are some very special stars out there. Many are in binary systems and do very weird things (and I have not covered the extremely rare ones such as Helium or Carbon stars). Variable stars are quite simple to explain in general and can be used for very important distance calculations.