StarDate

A pair of big, young star clusters is one of the skywatching highlights of autumn evenings. Each cluster contains thousands of stars. And hundreds of those stars are especially impressive – supergiants that are many times the size and mass of the Sun, and many thousands of times brighter.

The clusters are NGC 869 and 884. Together, they’re known as the Double Cluster. They’re in the northeast at nightfall, in the constellation Perseus the hero. Under dark skies, they’re visible to the eye alone as a single smudge of light. Binoculars split them apart, revealing two sparkling collections of stars.

The clusters are more than 7,000 light-years away. They form the core of the Perseus OB1 Association – a huge complex of stars and clouds of gas and dust. The region has given birth to perhaps tens of thousands of stars.

All of the stars were born in the recent past, as measured on the astronomical time scale. The Double Cluster is only about 14 million years old, compared to four-and-a-half billion years for the Sun. One way astronomers determine that age is through the types of stars found in the clusters. The most-massive stars burn out in a hurry. But the clusters still contain many such stars.

Some of those monsters won’t last much longer, though – they’ll blast themselves to bits as supernovas – brilliant light shows in the Double Cluster.

Script by Damond Benningfield

Cassiopeia is an easy constellation to pick out. The stars of the queen form a prominent letter M or W, which is visible even through moderate light pollution. And under dark skies, binoculars reveal several star clusters around that pattern, all immersed in the glow of the Milky Way.

One of the highlights is NGC 7789. It was discovered in 1783 by Caroline Herschel – the first woman to be paid for her work as an astronomer. Through a telescope, the contours of the cluster and nearby clouds of dust resemble a rose. So NGC 7789 also is known as Caroline’s Rose.

The cluster is about 6800 light-years away, in the opposite direction from the center of the galaxy. And it’s packed with stars. It has 1300 confirmed members, with an estimated total of 3,000 stars or more.

The cluster’s age has been tough to pin down. A recent study says it’s one-and-a-quarter billion years old. Previous estimates are a little higher. Regardless of the exact age, the cluster has made several orbits around the center of the galaxy. By the time many other clusters have made that many orbits, they’ve been pulled apart by the gravity of the rest of the galaxy. The stars survive, but they head their separate ways.
Caroline’s Rose is in the northeast at nightfall, just above Cassiopeia’s outline. It climbs high across the north during the night.

Script by Damond Benningfield

A quiet monster lurks in the heart of M31, the Andromeda Galaxy. Known as M31-star, it’s a black hole a hundred million times the mass of the Sun, and 25 times heavier than the black hole at the heart of the Milky Way.

Every large galaxy probably has a supermassive black hole. And many of them are quite bright.

The black hole itself is completely black – it swallows up all the matter and energy that falls into it, and nothing can come back out.

But the black hole’s gravity pulls in huge amounts of gas and dust, along with an occasional asteroid, star, or other large object. This debris forms a disk around the black hole. As material in the disk spirals closer to the black hole it gets extremely hot, so it shines brightly.

But M31-star is one of the quietest of all supermassive black holes – its disk produces very little energy.

That tells us that the black hole is not actively “feeding” – not much debris is falling into it. A distant ring of gas and dust encircles the black hole, with filaments of material spiraling into the disk. But the “feeding” rate is low, so M31-star remains quiet – a dark monster in the heart of M31.

Under dark skies, the Andromeda Galaxy is visible to the naked eye. It looks like a faint smudge of light that’s wider than the Moon. Right now, it’s about a third of the way up in the east-northeast at nightfall, and climbs high overhead later on.

Script by Damond Benningfield

Two-and-a-half million years ago, Earth was going through some big changes. It was the start of the Pleistocene epoch – a period of major ice ages. And the ancestors of modern humans were just appearing.

That timescale puts a little perspective on the distance to M31, the Andromeda Galaxy – the closest major galaxy to the Milky Way. It’s two-and-a-half million light-years away. So when we look at the galaxy, we’re seeing it as it looked two-and-a-half million years ago.

Andromeda and the Milky Way are much alike. Both of them are barred spirals – they have a long bar of stars in the center, with spiral arms wrapping off the ends of the bar. They’re about the same mass – roughly one trillion times the mass of the Sun. Andromeda appears to be wider, although it’s hard to measure the size of the Milky Way from our location inside it.

Andromeda isn’t giving birth to as many stars as the Milky Way is. That may be because it collided with a smaller galaxy in the distant past. The collision triggered a huge wave of starbirth that used up most of the star-making material – limiting the future population of the Andromeda Galaxy.

M31 is well up in the east-northeast at nightfall. Under fairly dark skies, it looks like a faint smudge of light wider than the Moon – the most-distant object that’s easily visible to the eye alone.

We’ll have more about the Andromeda Galaxy tomorrow.

Script by Damond Benningfield

The Moon has had a busy social calendar this week. It’s had meet-and-greets with the twins of Gemini and the planets Mars and Jupiter. And it has one more engagement tomorrow. Regulus, the heart of the lion, will stand above the Moon at dawn.
In centuries past, people took a more literal view of lunar social activities. Many thought the Moon was inhabited – including some astronomers. In 1778, for example, William Herschel reported seeing towns on the lunar surface. Herschel was one of the top astronomers of the time – he discovered the planet Uranus – so people took him seriously.
The idea of lunar life was especially common in the 19th century. In 1835, the New York Sun published a series describing a world teeming with life, including a civilization of “bat people.” The Sun attributed the discoveries to Herschel’s son, John – another famous astronomer. The whole thing was a hoax. But the idea of lunar life was so ingrained that the public ate it up.
In 1838, another writer described seeing structures built by “lunarians.” And in 1901, H.G. Wells wrote about intelligent life in “The First Men in the Moon” – a race of insect-like creatures called Selenites.
Today, we know that the Moon is devoid of life – at least for now. China and the U.S. have plans to establish permanent outposts on the Moon in the decades ahead – “lunarians” from right here on Earth.

Script by Damond Benningfield