Star Answers Again

by Deb on March 9, 2012

Stars Again

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1.     Sirius is the brightest star in the night sky. It was actually very important in setting the Egyptian calendar because it is prominent and easily spotted. It is so bright because of a combination of closeness and luminosity – it is only 8.6 light years away and bright white. It is part of the constellation Canis Major, the dog associated with Orion. It is actually a binary star, with a small white dwarf companion known as the Pup.

Sirius_A_and_B_Hubble_photo

Sirius A and B (Pup), bottom left. Photo from Hubble. The rays are an artefact of the imaging system.

2.     The closest known star to the sun is Proxima Centauri. It is a red dwarf only 4.2 light years away. A light year is the distance light travels in a year, so Proxima Centauri is about 40,000,000,000,000km or forty trillion kilometres away. It isn’t going to be a place we can go any time soon, but various types of nuclear propulsion may make it possible to get the flight time down to a century or so. It is possibly linked to the bright stars that form the binary Alpha Centauri. They orbit each other as a binary pair, and Proxima Centauri possibly orbits around both of them.

3.     A Pulsar is a pulsating star. Except it isn’t really, it’s spinning – when they were discovered and named no-one knew what was causing the regular bursts of electromagnetic radiation. When a massive star dies it can cause a supernova, which collapses into a neutron star. They keep the spinning energy they had before the explosion, but now they are tiny so they spin incredibly fast. They have strong magnetic fields, and the fast spin produces an electric field. This creates a beam of electromagnetic radiation blasting out from the magnetic poles.

Pulsar schematic

Courtesy of Roy Smits

The magnetic poles don’t necessarily line up with the rotational axis, so the beam sweeps around as the star spins, like a lighthouse. When it passes across where we can detect it we see it as pulses.

4.     The border between star and planet is where brown dwarfs live. Throughout the universe there are immense clouds of gas that condense through gravity. When they get massive and are packed tightly enough nuclear reactions may begin, which is what makes a star. Pulling matter in makes it swirl, like a whirlpool in your bath, and forms an enormous turning disc. Depending on when or where they are forming, these gas clouds may have many heavier elements that clump in their own little swirls in the disc to form dust or ice, then rocks, and eventually may become planets around a central enormous star.

But what about smaller gas clouds? Ones that are big enough to pull together but don’t get dense enough for a fusion chain reaction to keep going? These are the brown dwarfs – they form independently, not as swirls in another star’s dust, but are not big enough to ignite. They are generally between 13 – 75 times the mass of Jupiter,  but there is overlap with large planets and small stars. Planets larger than 13 Jupiters that form with a parent star are generally classed as planets, and stars smaller than the largest brown dwarfs that manage fusion are red dwarf stars. See the video below.

Relative star sizes

5.     The photo is Albireo, a triple star system that is visible with a telescope as a double star. The two stars are far apart and take about 75,000 years to orbit each other, which makes it easy to spot them. The yellow star is itself a binary system, but they are so close they can’t be separated by sight. That photo is from the Astronomy Picture of the Day site, which is definitely worth checking out.

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