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Weird Stars

Non-spherical Achernar

Achernar is a lovely southern hemisphere binary star 139 light years away.

It appears as the 10th brightest in the night sky, and is the hottest and bluest of these ten stars.

Achernar A is a B-type star of 7 solar masses. It spins so fast it's very not round, with radii 7.3 and 11.4 solar radii (56% greater). This distortion means its surface temperature also varies, from more than 20,000 K at the poles to 10,000 K at the equator (average 15,000 K). It is the least spherical star known.

Achernar B is an A-type star, and is about 12 astronomical units (AU, the average radius of Earth's orbit) away from Achernar A. (Compare with Saturn, which averages 9.5 AU.)

Red Sirius

Sirius is a binary pair that is the brightest star in the night sky, and has a mysterious history of changing colour. Read more on the Red Sirius page.

Neutron stars

A neutron star is a very dense star where the protons and electrons have been squished together, so neutrons are pretty much the only thing remaining.

A typical neutron star is between 1.4 and 3 solar masses with radius of about 12-13 km and a surface temperature of about 600,000 K. They are what is left after a large star (>10 solar masses) goes supernova.

Neutron stars are very dense, about 4x1017 kg/m³ (400,000,000,000,000,000 kg/m³). Because they are so dense, they have a very high escape velocity – perhaps half the speed of light. A neutron star's gravity bends light, so more than half of the surface can be seen.

As a star collapses it spins faster. Conservation of angular momentum applies to anything contracting while spinning, from an ice skater pulling in their arms to water going down a plughole. Neutron stars have contracted so much they can spin very quickly, producing a lighthouse-like object called a pulsar. The fastest known pulsar spins at 716 times per second.

Pulsars make finding exoplanets quite easy, because even small orbiting bodies cause a detectable Doppler shift in the pulses. The smallest object detected outside the solar system is a Moon-sized object orbiting a pulsar.

Neutron stars that are not pulsars are very hard to detect.

Black holes

Black holes are former stars that have collapsed so much that their surface gravity is so strong that not even light can escape.

The Schwarzschild radius is the radius of a sphere such that, if all the mass of an object were to be compressed within that sphere, the escape velocity from the surface of the sphere would equal the speed of light. An object where the mass is within its Schwarzschild radius is a black hole.

The surface at the Schwarzschild radius acts as an event horizon. Neither light nor particles can escape through this surface from the region inside.

The Schwarzschild radius of an object is proportional to its mass (and therefore volume) but the physical size of the object (for a given density) is proportional to the cube root of its volume.

The Schwarzschild radius of a body is proportional to its mass and therefore to its volume, assuming that the body has a constant mass-density. In contrast, the physical radius of the body is proportional to the cube root of its volume. Therefore, as the body accumulates matter, its Schwarzschild radius will increase more quickly than its physical radius.

A neutron star's Schwarzschild radius will exceed its physical radius at about 3 solar masses, turning it into a black hole.

Object Schwarzschild radius
Mt Everest < 1 nm
Earth 9 mm
Sun 3 km
Supermassive black hole at our Galactic Centre 13.3 million kilometres
Milky Way 0.2 light years
Observable Universe's mass 13.7 billion light years

A difference in gravitational attraction from the Moon and Sun causes tides on Earth. If the Moon got too close to Earth the difference in gravitational attraction from Earth would rip the Moon apart, and we would be left with a prominent ring around Earth and stagnant harbours. With a small black hole the difference in gravitational attraction when matter enters is enough to rip the matter itself apart, releasing large amounts of X-rays.

However, the larger an object and the further away it is from the source of the gravity, the smaller the difference in gravitational attraction is. This is why the Moon has a bigger effect on tides than the Sun does, even though the Sun's gravity is much greater. A really large black hole has a very small difference in gravitational attraction near its Schwarzschild radius, so crossing the Schwarzschild radius would not be noticeable. But it would still be a one-way journey.

The photon sphere is a spherical shell around a black hole where gravity is strong enough that photons are forced to travel in orbits. The photon sphere has a radius of 1.5 times the Schwarzschild radius. Stable orbits are not possible inside this radius, and any free fall object coming within the photon sphere is lost, spiraling in to the black hole, but a constant acceleration would allow a space probe to hover above the event horizon.

Super massive black holes appear to be at the centre of all large galaxies, and are believed to play a role in helping form the galaxies. At the centre of the Milky Way is a black hole called Sagitarius A* (pronounced "A-star"). We cannot see it directly but can observe other objects in tight fast orbits around it.