Monday, 27 June 2016

Hunting White Dwarfs, the Night’s Stellar Peewees

Hunting White Dwarfs, the Night’s Stellar Peewees:

Take a trip down the rabbit hole to the weird and weighty world of planet-sized white dwarf stars.

Earth compared to some of the most familiar white dwarf stars: Sirius B, the companion to Sirius; Procyon B; and Van Maanen's Star. While Earth-sized, each is extremely massive. Sirius B matches the Sun's mass, while the other two are about three-quarters as massive.

Bob King; Earth photo: NASA

A few weeks back we looked at some of the largest stars known. I hope you've had time and clear skies to make their acquaintance. Today, we proceed in the opposite direction and seek the smallest stars accessible to amateur telescopes: white dwarfs. Unlike the gasbag supergiants, white dwarfs pack their matter tightly, squeezing in a Sun's worth of mass into a fiercely hot sphere only as big as the Earth.

A spoonful of matter taken from anywhere in a supergiant except its compact core would only weigh a fraction of a gram. But that same spoon dipped into a white dwarf would weigh 5.5 tons and require a well-anchored crane to lift!

White dwarfs mark the end of the road for main sequence stars up to 8 times as massive as the Sun. During its life, a star burns through its hydrogen reserves, steadily converting that element to helium in its core.

Helium is heavier than hydrogen; as it accumulates, the core contracts and grows hot enough to burn helium into carbon. Carbon combines with helium to make oxygen.

Low-mass stars (fewer than 8 solar masses) like the Sun evolve into red giants before casting off their atmospheres. The former core, now collapsed, compressed, and extremely hot, becomes a white dwarf. UV radiation from the dwarf excites the former atmosphere, causing it to glow as a planetary nebula.

NASA

As the star transitions through phases of hydrogen and helium burning, it expands into a red giant, then puffs away its outer atmosphere, exposing a tiny core of carbon and oxygen. Because the star lacks the mass — and the heat and pressure that mass brings to bear — burning halts at oxygen. Without a "fire in its belly" to counteract the unrelenting force of gravity, the core is crushed into a planet-sized sphere with a temperature of over 180,000° F (100,000° C). A white dwarf is born!

The star might continue to crush itself into an even smaller object, but electrons in the carbon and oxygen atoms move to higher orbits and pick up speed during the contraction, resisting a potential implosion. It's called electron degeneracy pressure, and white dwarfs are said to be made of degenerate matter.

The bipolar planetary nebula Hubble 12 in Cassiopeia glows in the light of its white dwarf central star. Over time, the nebula will disperse and the dwarf will cool to become a black dwarf.

NASA / ESA / Josh Barrington

Often, the star's former outer layers glow in the copious ultraviolet light streaming from the dwarf, creating a colorful and jewel-like planetary nebula. The planetary expands and fades from view over a period of 20,000 to 50,000 years, leaving only a tiny, white-hot glowing ember that steadily grows cooler until it fades to become a black dwarf. This will almost certainly be the fate of our Sun some 6 billion years from now when it embarks upon a life of electron degeneracy. No need to go looking for any black dwarfs just yet. Since it takes something like a trillion years for a white dwarf to go black, our universe is far too young to have created its first.

Brilliant Sirius and its white dwarf companion Sirius B.

NASA / ESA

White dwarfs may be white and hot, but their small size means that nearly all are faint. But lucky for us, not too faint. The brightest and most familiar is Sirius B at magnitude +8.5. Even at its maximum separation of 11.5″ in 2025, this dwarf's a tough nut because of the overwhelming glare of Sirius itself. Likewise for Procyon B, which shines at magnitude +10.7 but hides in the glow of its primary star only 4.3″ away.

Omicron² Eridani B at magnitude +9.5, best viewed in the fall and winter months, forms an attractive double with a red dwarf star. It's probably the only white dwarf most amateurs have seen outside of several faint ones occasionally visible in the veiled centers of planetary nebulae.

Let's see if we can rectify that and add a few more of these exotic stars to your treasure chest. I've included charts and information below to help you find eight white dwarfs currently visible in the summer sky. They range in magnitude from about +11.5 to +12.5, making them all fairly easy to spot even in a 6-inch scope under dark skies.

If you need more, download Willem Luyten's White Dwarf Atlas which lists 96 white dwarfs and includes a photo for each. When using the atlas, be sure to precess the given epoch 1950.0 coordinates to 2000.0 using this handy coordinate calculator.

Van Maanen's Star

This chart shows the location of one of the brighter white dwarfs, Van Maanen's Star in Pisces. Each chart has its own degree scale, a bright star or two to help you get oriented, and stars plotted to magnitude +12 to +13. You'll find a direction indicator in the upper right corner and the star's R.A. and Dec. in the black box. Click the image for the large version, then right-click to save and print out for use at the telescope.