The Weirdest Objects in the Universe

The Weirdest Objects in the Universe

General Relativity Premise: acceleration and gravity are equivalent (i.e., are you accelerating or falling in a gravitational field?) Since the speed of light is always the same, this means that the path of light can be bent by gravity.

Implication: Gravitational Lensing One consequence of general relativity is that light must bend when it goes through a gravitational field.

Escape Velocity According to Newton, the greater the gravity, the faster an object must go to escape into space. This is called the escape velocity.

Escape Velocity Obviously, the escape velocity from any body depends on its mass, and on the starting distance. The further away you are, the weaker the gravity; the closer you are, the greater the gravity.

Black Holes If the gravitational attraction is large enough (either through high mass or small distance), there will be a place where the escape velocity is greater than the speed of light. This is a Black Hole.

Gravity and Light G M1 M2 F =  r 2 Remember: the force of gravity around black holes is the same as that on Earth. At large distances, nothing changes. If the Sun were a black hole, the Earth’s orbit would be the same as it is now. The only difference is that you can get a lot closer to a black hole. (In other words, r can get real small.) 4000 mi

Black Hole Sizes 2 x 10-26 cm The size (i.e., the radius of the event horizon) of a black hole depends only its mass. 1 cm 3 km

Event Horizon The radius at which the escape velocity is greater than the speed of light is called the event horizon (sometimes called the Schwarzschild radius). Anything beyond the event horizon will never return to our universe.

Gravitational Redshift of Light Close to a black hole, gravity is strong. According to relativity: • High Gravity  Large Acceleration • Large Acceleration  High Speed • High Speed  Time Dilation Time slows down (as you measure it) for someone close to a black hole). This includes atoms – the frequency of emitted light gets smaller. Thus produces a gravitational redshift. It also means that for an object at the event horizon, time stands still (at least, as you measure it).

Warping of Space-Time Another way to look at the relation between black holes and light is to assume that light travels in straight lines, but that mass warps space-time. Orbits (and light) just follow the curve.

A black hole represents the extreme case where gravity punches a hole in space-time.

Black Holes, Neutron Stars, and Tides Remember – gravity depends on mass and distance. Objects such as neutron stars and black holes are very small, yet very massive. So if you get close, the tides may get you!

Binary Star Evolution About half the stars in the sky are binaries. These stars may begin life as separate entities, but often times this does not last.

Roche Lobes Between any two stars are gravitational balance points, where the attraction of one star equals the attraction of the other. The point directly between the stars is called the Lagrange point. The balance points in general map out the star’s Roche Lobe. If a star’s surface extends further than its Roche Lobe, it will lose its mass.

Binary Star Classification Detached: the stars are separate and do not affect one another Semi-detached: one star is spilling mass (i.e.,accreting) onto the other Contact: two stars are present inside a common envelope (i.e., it is a common-envelope binary).

Common Envelope Evolution If a red giant overflows its Roche lobe so that it engulfs the companion, its outside may be stripped away, leaving only its hot core.

Common Envelope Evolution If a red giant overflows its Roche lobe so that it engulfs the companion, its outside may be stripped away, leaving only its hot core. This object will look like an (odd-shaped) planetary nebula

Accretion If a star overflows its Roche lobe through the Lagrange point, its material will simply go into orbit about the companion. The material will stay in the plane of the system and form an accretion disk.

Accretion • According to Kepler’s laws, matter close to a star will orbit faster than material further away. If there’s a lot of material in a disk, this will cause the atoms will rub up against each other. There will be friction! So • The material will lose orbital energy and spiral in • The disk will get real hot.

Accretion • According to Kepler’s laws, matter close to a star will orbit faster than material further away. If there’s a lot of material in a disk, this will cause the atoms will rub up against each other. There will be friction! So • The material will lose orbital energy and spiral in • The disk will get real hot. The faster the gas moves, the greater the friction, and the hotter the disk. If the companion star is compact (white dwarf, neutron star, or black hole), then near the center, the disk will emit x-rays!

X-ray Identifications Because accretion disks around compact objects can get much hotter than stars, x-ray surveys can identify them! Optical Picture X-ray Picture The more compact the object, the hotter the accretion disk, and the more (very high energy) x-rays that are produced.

Novae By definition, white dwarfs are what they are because they have no more fuel to burn. But if a white dwarf accretes hydrogen, it suddenly will have fuel, and can burn it – explosively. This is called a nova. Outbursts can occur once every few years, or once every 50,000 yr, depending on the system. When in outburst, a nova will be as bright as 500,000 L.

Type Ia Supernovae Recall that white dwarfs are held up by electron degeneracy. Their masses must therefore be less than 1.4 M. Over time, accretion may push a white dwarf’s mass over this limit. If this happens, the star will collapse, and become a Type Ia Supernova. A Type Ia supernova is just as bright as a regular (Type II) supernova, but it doesn’t leave behind a remnant. Models suggest that the star is totally destroyed.

Millisecond Pulsars When a star explodes as a supernova, the neutron star that is left behind rotates about once a second. However, if a star accretes onto this neutron star, it can cause it to spin 1000 times faster!

Evaporated Stars Accretion disks around neutron stars (or black holes) emit large numbers of very energetic x-ray photons. These x-rays can strike the companion star’s atmosphere, and heat it up so much that the star literally evaporates. All that remains may be some rubble around a bare millisecond pulsar.

Accretion Disks and Black Holes The accretion disk around a black hole can extend very close to the event horizon. The gas speed there is very close to the speed of light, so the friction in the disk is extremely intense. This type of disk will produce the most-energetic x-rays. But note: in the optical, the disk may be faint – much fainter than the companion (donor) star.

Finding a Black Hole • Identify the optical counterpart of an x-ray binary • Observe the optical component of the binary • Estimate the total mass of the system using Kepler’s and Newton’s laws • Estimate the mass of the visible star from its spectral type, etc. • Subtract to estimate the mass of the unseen companion • Exclude possible stellar types based on visibility and knowledge of stellar astrophysics If all possibilities are excluded, you have a black hole!

Next time -- Star Formation

The Weirdest Objects in the Universe