StarDate

Andromeda is best known for the Andromeda Galaxy, M31. But that’s not the constellation’s only good “deep-sky” object. One of them is inside our own galaxy: a dying star known as the Blue Snowball Nebula.

Even though it’s much closer than the Andromeda Galaxy, the Snowball is still a good jaunt from Earth – about 5500 light-years. It’s a bubble of gas and dust expelled by a star at the end of its life.

The star’s nuclear reactor – its hot core – is switching off. Radiation from the core is blowing away the star’s outer layers. That bubble spans close to a light-year, and it’s expanding at about 60,000 miles per hour. The gas is “energized” by ultraviolet light from the core, causing it to glow. Seen through a telescope, it looks like a blue snowball.

With the outer layers barreling away from it, the core is exposed to view. It’s tiny – only about 10 percent the diameter of the Sun. But it’s much, much hotter than the surface of the Sun, so it shines thousands of times brighter.

Before long, though, the core’s nuclear fire will be extinguished. The core will continue to shine, but at a tiny fraction of the Sun’s brilliance. With no energy source to light it up, the blue snowball will melt away.

Andromeda is low in the east and northeast as night falls. The nebula is at the right edge of the constellation, not far to the left of the Great Square of Pegasus.

Tomorrow: a faint eclipse.

Script by Damond Benningfield

The flying horse has lost his belly button. Astronomers took it away from him a century ago.

It’s represented by the star Alpheratz. For centuries, the star was shared by Pegasus the flying horse, and Andromeda the princess. It was his navel, and her head. It was the brightest member of both constellations.

But in the 1920s, astronomers drew formal boundaries for all the constellations. So each star was assigned to a single constellation – no more sharing. Alpheratz was just on the Andromeda side of the line, so the princess kept the star.

In a way, though, the star kept its dual identity. That’s because it’s actually two stars locked in orbit around each other. But they’re quite close together – closer than Earth is to the Sun. So from their distance of almost a hundred light-years, their light blurs together to form a single point.

Both stars are bigger, more massive, and brighter than the Sun. The higher mass means they’ll live billions of years less than the Sun will. They’re also much hotter than the Sun, so both stars look almost pure white, compared to the yellow hue of the Sun.

Alpheratz is about a third of the way up the eastern sky as night falls. It’s at the left corner of the Great Square of Pegasus, which is tilted as it rises. The rest of the flying horse stretches to its right and upper right. Andromeda stretches to the lower left – the constellation that won the right to the star Alpheratz.

Script by Damond Benningfield

Earth faces a non-stop barrage of particles from a supermassive black hole at the heart of a giant galaxy. Fortunately, it’s almost a billion light-years away – not close enough to cause any problems.

BL Lacertae is in Lacerta, the lizard. The constellation is in the east-northeast at nightfall, to the left of the Great Square of Pegasus. It passes high overhead around midnight. Its stars are all so faint that you need fairly dark skies to see any of them.

At first, BL Lac appeared to be one of those faint stars – much too faint to see without a telescope. It was discovered almost a century ago. It gets brighter and fainter over a period of a few days, so it was classified as a variable star.

In the late 1960s, though, astronomers realized that BL Lac is far outside the Milky Way Galaxy. It’s a beam of energy from a black hole at the center of a giant galaxy.

Gas funnels into the black hole. But powerful magnetic fields channel some of that material back into space. It forms “jets” of charged particles that beam into the universe at close to the speed of light. And one of those jets takes dead aim at Earth.

BL Lac is the prototype of a whole class of black holes with jets aimed our way. In fact, it even gave the name to a new category of objects. The name combines BL Lac with “quasar” – a similar type of object – to make “blazar” – supermassive black holes blazing across the universe.

Script by Damond Benningfield

A massive star in a nearby galaxy may have winked out of existence with little or no pyrotechnics. Instead of blasting itself to bits as a supernova, leaving behind a neutron star or black hole, it skipped the “blasting” stage. Instead, the entire star collapsed to form a black hole.

VFTS 243 is a binary system in the Large Magellanic Cloud, a companion galaxy to the Milky Way. One member is a star that’s about two dozen times as massive as the Sun, and more than 150,000 times brighter. The other is a black hole about 10 times the Sun’s mass.

A team of astronomers looked for evidence that the star that formed the black hole had exploded as a supernova – the likely fate of most stars at least eight times the Sun’s mass. The supernova forms when the star’s core collapses and its outer layers fall inward, then rebound.

Such a blast would leave a cloud of debris. It might also give the black hole a strong “kick” away from the companion star. The astronomers found no evidence of either. There was no debris from an explosion. And the bright star and the black hole follow an almost circular orbit around each other. That suggests there was little or no interaction between them when the black hole formed.

Almost the entire mass of the star must have collapsed to form the black hole. So the star that give birth to the black hole in VFTS 243 might simply have vanished from the sky.

Script by Damond Benningfield

The phrase “taking the plunge” often suggests a combination of excitement and risk: making an investment, getting married, or literally jumping off a cliff into a pool of water. But for matter orbiting a black hole, taking a plunge is fatal – it’s entering a zone where it’s doomed to fall into the black hole, from which it can never escape.

A team of astronomers recently published the first solid evidence of this “plunging zone.” The zone is outside the black hole’s event horizon, which is the boundary between the black hole and the rest of the universe. But it’s closer than the distance at which matter can remain in a stable orbit. It’s like a river: The stable orbits are like a steadily flowing river, while the plunging zone is the waterfall at its end.

The team analyzed X-ray observations of a system about 10,000 light-years from Earth. The system consists of a star that’s about half the mass of the Sun, plus a black hole that’s roughly eight times the Sun’s mass. The black hole pulls gas from its companion. The material spirals around the black hole, forming a wide disk.

Albert Einstein’s theory of gravity predicts that matter at the inner edge of the disk should enter the plunging zone. In that zone, light can still escape, but matter will fall through the event horizon. Some of the X-rays observed by two space telescopes were coming from this zone – the first sighting of the plunging zone.

Script by Damond Benningfield

When a massive star dies, it probably blasts itself to bits as a supernova. All that’s left is its tiny but heavy core – a neutron star or black hole. But astronomers aren’t quite sure which stars are likely to end up in the two categories.

All stars shine by “fusing” elements in their cores to make heavier elements. Stars like the Sun convert hydrogen to helium. But the heaviest stars continue making new elements all the way up to iron. At that point, the core collapses. The outer layers fall in, then rebound in spectacular fashion – as a supernova.

The core’s collapse might stop when it forms a neutron star – an extremely dense ball only a few miles in diameter. But it might not stop – the core might collapse even more, forming a black hole.

The dividing line isn’t clear – astronomers aren’t sure which stars will form neutron stars, and which will make black holes.

There also appears to be a gap in those dead stars. The heaviest neutron stars yet seen are about twice the mass of the Sun. The lightest black holes, on the other hand, are more than three times the Sun’s mass. That could simply mean that we just haven’t found anything between them – neutron stars and black holes are hard to study. But it certainly means that scientists have more work to do to explain the fates of some of the heaviest stars in the universe.

Some massive stars might form black holes without exploding; more about that tomorrow.

Script by Damond Benningfield

Antares is a big loser. The star has lost enough gas to make one or more stars as massive as the Sun. And it’s destined to lose a lot more. It’ll explode as a supernova, blasting its outer layers into space. That could expel enough material to make 10 Suns or more.

Antares is a supergiant – one of the bigger, brighter stars in the entire Milky Way Galaxy. If it took the Sun’s place in our own solar system, it would gobble up the four innermost planets – including Earth.

The star is so big because it’s heavy – probably a dozen times the mass of the Sun or more. That makes its core extremely hot. Radiation from the core pushes on the surrounding layers, inflating the star. And much of the gas at its surface just keeps on expanding – it escapes from Antares and flows out into space.

Even though Antares is less than one percent the age of the Sun, its time is about up. Within the next million years or so, the nuclear reactions in its core will shut down. The core will collapse, while its outer layers explode. The core probably will become a neutron star – a super-dense ball only a few miles across. But it’s possible that it could collapse even more – becoming a black hole. More about that tomorrow.

Look for Antares near the Moon the next couple of evenings. Tonight, it’s close to the upper left of the Moon at nightfall. It’ll be about the same distance to the right of the Moon tomorrow night.

Script by Damond Benningfield

With summer just about ready to give way to fall, one of the signature star patterns of the new season is climbing higher into the evening sky. The Great Square of Pegasus is in the east at nightfall. It’s tilted as it rises, so it looks more like a diamond than a square.

The jewel at the top of the diamond is Beta Pegasi. It’s a red giant – an old, bloated star that shines reddish orange. That color isn’t visible to the unaided eye because the star isn’t all that bright – mainly because it’s about 200 light-years away. But the tint should be easy to see through binoculars.

A star’s color is the result of its surface temperature. Hot stars are blue-white, while cool stars shine orange or red. Beta Pegasi is thousands of degrees cooler than the Sun. That’s because the star is nearing the end of its life. Changes in its core have caused its outer layers to puff up, so the star is almost a hundred times the Sun’s diameter. That’s caused the surface to get cooler – and redder.

The change in temperature also produces a change in the form of energy the star radiates into space. Stars like the Sun produce most of their energy at visible wavelengths. But most of the light from Beta Pegasi is in the infrared – wavelengths that are too long for human eyes to see. When you add all the wavelengths together, Beta Pegasi shines about 1500 times brighter than the Sun – a red jewel sparkling across the galaxy.

Script by Damond Benningfield

Charles Messier was a comet hunter. But he kept coming across annoying objects that resembled comets. So he compiled a catalog of them to keep both himself and others from wasting time – more than a hundred in all. And 250 years ago tonight, while studying a comet in Cassiopeia, he discovered object number 52 – a star cluster that’s thousands of light-years from Earth.

Astronomers have been studying Messier 52 ever since then. Yet they still don’t know all of its details. That includes its precise distance. Measurements made in recent years have ranged from less than 5,000 light-years to more than six thousand. And without knowing the cluster’s distance, it’s hard to nail down its other characteristics. Its age, for example, could range from 120 million to 160 million years.

A study a few years ago counted 369 stars in the cluster, although the total’s probably much higher. Many of the stars are crowded in its middle. But some of them are dozens of light-years from there. At the cluster’s edge, stars can be pulled away by the gravity of the rest of the galaxy – reducing the size of this family of stars.

M52 is well up in the northeast at nightfall. It’s above the sideways letter W formed by some of Cassiopeia’s brighter stars. The cluster is too faint to see with the eye alone, but it’s an easy target for binoculars.

Tomorrow: A ruby jewel atop a stellar diamond.

Script by Damond Benningfield

In most star systems, Spica B would be a monster. It’s about seven times the mass of the Sun, and more than 2,000 times brighter. Its heft puts it in the top one-tenth of one percent of all the stars in the Milky Way Galaxy.

But Spica B has a companion that’s far more impressive. Spica A is more than 10 times the Sun’s mass, and 20 thousand times its brightness. And it has a more impressive fate: It’ll explode as a supernova.

On the other hand, Spica B will live tens of millions of years longer than Spica A. That’s because of the difference in mass. Heavier stars burn through their nuclear fuel more quickly; the heavier the star, the faster and hotter it burns. So while Spica A is quite young, it’s already completed the “prime” phase of life. Now, it’s getting bigger and brighter as it approaches the end.

After Spica A explodes, Spica B will still have another 60 million years or so left. Even so, its lifetime will be quite short – about 10 billion years less than the Sun. And when its time is up, it probably won’t explode. Instead, it’ll cast off its outer layers in a less-violent process. That will leave only its hot but tiny core, shining faintly for many billions of years to come.

Look for the Spica system quite close to the Moon as darkness falls. It looks like a single point of light. The planet Venus – the brilliant “evening star” – stands to their lower right.

Script by Damond Benningfield