Nights of the Living Dead — What Stars Leave Behind

Stars leave interesting messes after they die: diamond-studded puffballs, neutron stars, and black holes. We explore an example of each in June's night skies.

Still hungry at the table

The white dwarf star in the AE Aquarii system spends its retirement siphoning gas from a closely-orbiting companion.
Casey Reed / NASA

Stars live long lives, from supergiant powerhouses that exhaust their fuel in mere millions of years to the frugal dwarfs that can persist for trillions. From a human standpoint, they're around forever. But despite appearances, stars are hardly static. They flicker, bloat, shrink, and sometimes even explode, transforming elements in their hot cores into energy that sustains their heat and light.

When the fire runs its course and no fuel is left to beat back the crushing grasp of gravity, the star becomes transformed. It sheds what it no longer needs and begins a new life as a stellar ember. You and I, when we die, are disassembled into simpler things and fed back into the great web of earthly life. Many stars keep it together after death, so that we can stop by to see what remains after the party's over.

Outside of total destruction in certain types of supernovae or exotic mergers, most stars "die" in one of three ways after using up their fuel reserves. Normal, sun-like stars bloat into red giants, shed their outer envelopes, and expose their now super-compressed, hot cores. They evolve into Earth-sized white dwarf stars, their powerful ultraviolet rays setting their expanding shells aglow as planetary nebulae.

Door #1, Door #2, Door #3

A star's fate depends upon its mass. This illustration depicts how a small, large, or extra-large star may develop into white dwarf, neutron star, or black hole. Not to scale.
NASA / CXC / M. Weiss

Suns 8 to 40 times more massive than the Sun often end their lives dramatically as supernovae. During the star's collapse, the implosion can crush the core beyond white-dwarf density into a neutron star the size of a modest city. Protons and electrons merge into a sea of pure neutrons packed so tightly that two solar masses of material squeeze into a sphere between 6–12 miles (10–20 km) across. If more than three solar masses are kneaded into the collapsing core, the infall will continue until a black hole is formed.

Only white dwarfs are directly visible in modest amateur instruments. The brightest, Sirius B, shines at magnitude +8.3, though the easiest is 40 Eridani B at magnitude +9.5. Many more pop into view around magnitude +13–14 mark. No one's seen a black hole yet and all neutron stars are much too faint at visible wavelengths to see directly.

That doesn't mean you can't infer their presence by how they affect their environment, though. Some are surrounded by disks of swirling matter robbed from a nearby companion. As the material rubs together while funneling from the disk either onto the neutron star or into the black hole, friction heats the matter to billions of degrees, generating everything from X-rays to visible light.

Largest white dwarf

The white dwarf inside the Dumbbell planetary nebula, M27, shines at magnitude +13.5. It's the largest white dwarf known with a diameter of about 76,525 km (47,550 miles). You'll find it at the center of the nebula (tick marks) along an axis of nebular knots.
Jim Misti

Examples of all three stellar endpoints are visible in June skies. We'll visit with the evolving white dwarf within the Dumbbell Nebula (M27), the neutron star in one of the sky's brightest X-ray sources, Scorpius X-1, and the stellar-mass black hole V4641 Sgr in Sagittarius. V4641 Sgr, like Sco X-1, is surrounded by an accretion disk of glowing gas pilfered from a nearby companion.

There are no bright, solitary white dwarfs in the summertime night sky for mid-northern skywatchers, that's why I selected a planetary nebula instead. But if a singular dwarf is your thing, keep an eye on Van Maanen's Star in Pisces, now rising near Venus in the morning sky. Shining dimly at magnitude +12.4, it should come into better view in July.

Sagitta and M27

The little constellation Sagitta, the Arrow, is the key to finding the Dumbbell Nebula, M27 (which glows at 8th magnitude). Sagitta is located 10° north of bright Altair. M27 (top left corner here) is located 3.3° north of the point of the Arrow.

In the meantime, we'll look for our white-dwarf-in-waiting in the heart of one of the brightest and easiest to find planetary nebulae in the sky, the Dumbbell, in the constellation Vulpecula. Listed at magnitude +13.5, it's smack dab in the nebula's center; an 8-inch scope magnifying around 100–150× will pull it out of the surrounding nebulosity.

The predecessor star was a red giant that sloughed off its atmospheric baggage in strong stellar winds some 3,000 years ago. Leftover heat from the good old days when the star still burned nuclear fuel, combined with gravitational contraction, have heated the erstwhile core to 153,000°F (84,725°C), more than 15 times hotter than the Sun. Copious amounts of UV light emitted by the dwarf stimulate the gases in the nebula to glow like the neon sign at your favorite bar or pub.

With billions of years of time on its hands, the white dwarf gradually grows cooler until becoming a hypothetical black dwarf, the stellar equivalent of a cold, black ember. No one's ever observed a black dwarf because the universe is still too young to have produced any!

Our next star still around long after its expiration date, Scorpius X-1, was discovered during a sounding rocket flight in 1962. Also known by its variable star designation, V818 Sco, it's the most consistently powerful source of X-rays in the sky outside of the Sun. The source of all that energy is a neutron star with a mass 1.4 times that of the Sun siphoning gas from a closely orbiting donor star with just under half a solar mass.

Star-hopping to a neutron star

Finding Scorpius X-1 is a snap. Star-hop east from Beta Scorpius to the line of stars in southern Ophiuchus, then head back west to a neat equilateral triangle (outlined). Center your scope on the magnitude +8.5 star and then use the provided AAVSO chart with magnitudes (decimals omitted) noted. North is up and several key stars are shown with magnitudes. Click to enlarge.
Stellarium (left) and AAVSO

The material is pulled into a spinning accretion disk and ultimately falls to the surface of the neutron star. Because of the star's extreme gravity, the falling gas releases far more energy than it would through thermonuclear fusion. Heated to 180,000,000°F (100,000,000°C) the system emits gobs of light across the electromagnetic spectrum. Through our scopes we see this holy terror as irregular light fluctuations between magnitude +12 and +13.

Sco X-1 is what's left of a supernova that exploded some 30 million years ago. Through the telescope, it looks no different than a star but one that, if watched for several weeks, will show obvious variations in light as material within the accretion disk comes crashing down to the neutron star's surface. Bright guide stars make finding this exceptional object easy.

Some day soon we'll image the supermassive black hole located in the center of the Milky Way galaxy. Already, observations are underway using the Event Horizon Telescope — eight radio telescopes around the world linked together electronically. The last I read, the first image is expected in early 2018.

Black hole at work

An artist's concept of a microquasar or stellar-mass black hole like V4641 Sgr. The black hole is stealing gas from a companion star (left). The gas forms a thin, hot disk around the black hole. When enough gas builds up there is a bright flare-up of X-rays, and jets of charged particles squirt away at close to the speed of light. A similar disk of pilfered star-stuff orbits the neutron star Scorpius X-1.
ESA

While we wait, let's dig down into the Sagittarius Teapot  and set our sights on a faint "star" subject to wild and crazy light variations. V4641 Sagittarii is a stellar-mass black hole (vs. the hundreds of thousands to billions in the supermassive variety) hiding some 3–10 solar masses of material in plain sight more than 24,000 light-years away.

How do we know it's there? The system unashamedly announced its presence in 1999 with a sudden burst of powerful X-rays and a two-magnitude leap in visual brightness. For a time, it was the brightest X-ray source in the sky and studied by astronomers around the world using radio telescopes as well as the orbiting Rossi X-Ray Timing Explorer (RXTE). Flickering X-ray emission and powerful jets flinging particles into space at almost the speed of light pointed to a black hole about 11–37 miles (18–60 km) across. In a now familiar scenario, the black hole funnels material from a normal star into an accretion disk. Flare-ups within the disk create the light variations.

Hole in Sagittarius

Finding the site of V4641 Sagittarii, the site of a stellar-mass black hole, couldn't be easier. Identify the magnitude +6.5 star in the chart at left, then use the AAVSO chart to star hop from that star (arrowed) to the target. Both show north up. Click to enlarge and print out.
Stellarium (left) and AAVSO

V4641 Sgr may lie in a relatively crowded field but it's not too hard to find, since it's only 15 arcminutes north-northeast of a 6.5-magnitude star. Once you've centered your scope on the star, use the AAVSO chart to arrive at the black hole system. As I write, the V4641 Sgr has hovered between magnitude +13.3–13.5 for some time, but you never know when that might change. During the September 1999 eruption, it shot all the way up to magnitude +10.3. After declining back to the mid-13s, it surprised us all again in August 2003 with another rise to magnitude +11.5 followed by a similar outburst in June 2005.

So in the end, the end is not really the end for most stars. They put on Groucho Marx glasses and continue into their afterlives, cracking jokes and biding time while we dash around fretting about what's next.

4 thoughts on “Nights of the Living Dead — What Stars Leave Behind

  1. Rusty

    Very interesting reading, Bob. I’ll give these three a try when I attend the Golden State Star Party later this month. When sharing an image I captured of the Helix Nebula, I have explained the scenario of a star contracting due to gravity when its nuclear fuel runs out. Of course this is why the star becomes hot and expels its outer atmosphere. I didn’t realize that the dense star in a planetary nebula could eventually become a white dwarf.

    Thanks for the interesting information. It makes it all the more satisfying to understand what is going on with the things we observe.

  2. Jim White

    Thanks for the excellent article Bob. I like the way you describe what happens when the balance of a star’s crushing force of gravity, and the explosive force of the star’s nuclear reaction, comes to an end….”when the fire runs its course and no fuel is left to beat back the crushing grasp of gravity”

    I guess you could say I’m one dumbbell for which you’ve made the universe less nebulous….

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