StarDate

The Sun’s most-remote major planet is fascinating. It features the fastest winds in the solar system, and it once had a storm big enough to swallow Earth. Methane in its atmosphere gives it a beautiful blue color. And its biggest moon is dotted with giant geysers.

Yet Neptune — the fourth-biggest planet — has received only one visitor from Earth. Voyager 2 flew past it 30 years ago today. It discovered five moons and confirmed another. It also discovered four new rings around the planet. And it studied Neptune’s atmosphere and its magnetic field.

Neptune is about 30 times farther from the Sun than Earth is. At that distance, the Sun is only about a thousandth as bright as it is from Earth. So Neptune receives little heat from the Sun to power its weather.

Yet Voyager 2 discovered that Neptune’s atmosphere is dynamic — powered by heat from inside the planet. Voyager photographed a giant storm, known as the Great Dark Spot. Winds around it blew at more than a thousand miles per hour. And bright clouds that were close to it rose high above the rest of the atmosphere.

Voyager also looked at Neptune’s biggest moon, Triton. It found dozens of geysers of nitrogen ice. They deposited dark streaks that were miles long — something not seen on any other world in the solar system.

And Neptune is nearing its best showing of the year. It climbs into view in early evening, at the western edge of Aquarius. It’s so faint, though, that you need a telescope to see it.

Script by Damond Benningfield

Most of the constellations are tough to pick out. Their classical “connect-the-dots” outlines are so faint, small, or spread out that it’s tough to see them as pictures.

That’s not the case for two of the most prominent constellations of summer, though. Scorpius and Sagittarius are big and bright, and they’re easy to pick out. They’re in the south at nightfall on these late-summer evenings. Look for the curving body of the scorpion just above the horizon, with the orange star Antares in its middle. Sagittarius is to the left of the scorpion, with its brightest stars forming a teapot.

If you have a dark sky, away from city lights, look about halfway between them — between the scorpion’s stinger and the spout of the teapot — for a hazy patch of light that’s a bit bigger than the full Moon.

That’s Messier 7, a star cluster that’s close to a thousand light-years away. Its hundred or so stars were born from a single cloud of gas and dust, around 200 million years ago.

One reason the cluster stands out is that several of its stars are nearing the ends of their lives. That’s caused them to puff up to giant proportions. Such stars are hundreds of times brighter than the Sun, so they’re visible across great distances.

Binoculars will show quite a few of the cluster’s stars, while a small telescope will let you see most of them — a family of stars sitting between two prominent constellations of summer.

More about Scorpius tomorrow.

Script by Damond Benningfield

The face of Taurus, the bull, is doomed. The V-shaped face is outlined by a star cluster known as the Hyades. But the cluster is falling apart.

The Hyades is the closest major big cluster to the Sun — about 150 light-years away. All of its stars were born about 650 million years ago, from a giant cloud of gas and dust. Today, the stars all move through space together, as a single unit. But the cluster is less impressive today than it was in the past.

A recent study looked at observations of stars in and around the Hyades by Gaia, a space telescope. It’s measuring the positions and motions of a billion stars. It revealed 500 members of the cluster itself. But it also revealed two “tails” of stars near the cluster — one ahead of it, and the other behind. Together, they contain more than 500 stars.

The stars in these tails are about the same age as the stars of the Hyades. They also have the same chemistry, and they move with the same speed and in the same direction as the Hyades. That suggests the stars were members of the cluster in the past. They were kicked out by encounters with other stars, or by the gravitational pull of the rest of the galaxy.

So someday, the entire cluster may be pulled apart — destroying the face of the bull.

Look for the face at dawn tomorrow to the right of the Moon. The bright star close to the Moon is Aldebaran, the bull’s eye. Although it’s at one point of the V, it doesn’t belong to the cluster — it’s on its own.

Script by Damond Benningfield

Mergers between big galaxies are violent events. They can kick millions or billions of stars into intergalactic space, and dramatically alter the orbits of other stars. They can trigger the birth of millions of new stars — many of which will quickly blast themselves to bits. And they can result in the merger of two supermassive black holes — creating one of the brightest objects in the universe.

Most big galaxies have giant black holes in their cores. The one at the heart of the Milky Way, for example, is about four million times the mass of the Sun. And the black holes in other galaxies can be billions of times the Sun’s mass.

When two galaxies with such giant black holes come together, the black holes fall toward each other. They can stage a cosmic “dance” that lasts for hundreds of millions of years. As they loop around each other, they emit gravitational waves, causing them to move closer together.

As the black holes spiral closer, they stir up giant clouds of gas and dust. Some of this material is funneled toward the black holes. As it moves closer, it’s heated to millions of degrees. That creates a quasar — one of the more brilliant objects in all the universe.

The fireworks might not end when the black holes finally merge. Instead, the combined black hole might be kicked out of the merging galaxies. It can drag stars and gas clouds along with it — creating one final round of pyrotechnics as it heads for intergalactic space.

Script by Damond Benningfield

The Milky Way is headed toward a galactic pile-up. Our home galaxy is likely to merge with M31, our closest big neighbor galaxy. That will trigger the birth of millions of new stars. And it will fling the Sun and its planets into a new orbit — and perhaps out of the galaxy entirely. There’s no need to worry about it, though. It won’t start for another four and a half billion years.

M31 — also known as the Andromeda Galaxy — is about two and a half million light-years away. But it’s moving toward us. Every hour, it covers about a quarter of a million miles — roughly the distance between Earth and the Moon.

The Gaia satellite measured the precise motions of more than a thousand stars in M31. A team of astronomers used those measurements to come up with a better timeline of the encounter.

The team calculated that M31 will pass about 400,000 light-years from the Milky Way in four and a half billion years. The haloes of “dark matter” that envelop the galaxies will create friction. That’ll slow the galaxies down and pull them together.

They’ll pass through each other a few times. Each encounter will fling huge ribbons of stars out of each galaxy. There’s a chance the Sun could get caught in such a ribbon, carrying Earth along with it. The merger also will trigger intense bouts of starbirth — creating millions of new stars.

Eventually, Andromeda and the Milky Way will merge, creating a single monster galaxy — the final result of a galactic pile-up.

Script by Damond Benningfield

NGC 6052 is a mess. It looks a little like a pair of big spiders locked in mortal combat. Instead, though, it’s two galaxies that are staging their own combat — they’re in the process of merging.

The galaxies are about 230 million light-years away. They’re in the constellation Hercules, which is high in the western sky on August evenings. Both galaxies are spirals — cosmic pinwheels that are similar to our own galaxy, the Milky Way.

As the galaxies of NGC 6052 come together, gravity is throwing some of their stars out of the galaxies in giant ribbons. At the same time, clouds of gas and dust are slamming together to give birth to new stars. In fact, colliding galaxies create new stars at an amazing rate — hundreds of times more stars than the Milky Way is making.

Collisions between galaxies are common. So most of the galaxies in the present-day universe probably are the result of a series of mergers between smaller galaxies.

In fact, a study a couple of years ago found that we should be able to see up to two trillion galaxies. But many of those galaxies are puffballs that existed when the universe was young, and their light is just now reaching us. If we could take a snapshot of the universe right now, we’d see only a couple of hundred billion galaxies. Most of the early galaxies would have vanished — swallowed up in galactic mergers.

The Milky Way is headed toward its own big merger, and we’ll talk about that tomorrow.

Script by Damond Benningfield

We have lots of addresses. After your street address, city, state, and country, you might add lines for Planet Earth, the solar system, and the Milky Way Galaxy.

The Milky Way is just one of 200 billion galaxies or more in the present-day cosmos. Each one is an “island universe” — a vast collection of stars, gas and dust for making new stars, and mysterious “dark matter.”

Galaxies come in a few basic layouts. Many of them are wide, thin disks like the Milky Way. Waves traveling through the disks form beautiful spiral arms, so the galaxies look like pinwheels.

Others are shaped like fuzzy rugby balls. Their stars are old, and the galaxies don’t have much gas for making more stars. And still others are blobs with no easily definable shape.

There’s huge variety within these groups. Some galaxies are little more than puffballs. They’re only a few dozen to a few hundred light-years wide, and they contain only a few hundred million stars — a tiny fraction of the size and population of the Milky Way.

At the opposite end of the scale are the largest galaxies. They span a million light-years or more, and they host a hundred trillion stars — numbers that dwarf the Milky Way.

With so many galaxies around, it’s inevitable that they occasionally sideswipe each other, or even ram together head-on. In the end, the galaxies may merge, forming even bigger “island universes.”

We’ll talk about about one example of merging galaxies tomorrow.

Script by Damond Benningfield

Summer is the best time of year to seek out the Milky Way — the hazy band of light that outlines the disk of our home galaxy. If you can find an especially dark skywatching site as night falls, you’ll see it arcing from teapot-shaped Sagittarius in the south, through the swan high in the east, and over to W-shaped Cassiopeia in the northeast.

If you’d been around a few billion years ago, though, the view wouldn’t have been nearly as impressive. The galaxy’s disk might have contained only half as many stars as it does today.

Researchers looked at observations from Gaia, a spacecraft that’s been measuring a billion stars. The observations show how different types of stars are spread through the galaxy. That tells astronomers when and where the stars were born.

From that, the researchers concluded that the galaxy began giving birth to many stars about five billion years ago. By a couple of billion years later, the Milky Way might have been producing 10 times as many stars as it does now. In fact, that outburst could have produced half of the stars we see today.

The outburst might have been triggered by a merger with another galaxy. The merger would have brought in a lot of gas — the raw material for making stars. It also would have squeezed big clouds of gas, causing them to form stars.

Things have settled down over the last billion years or so. Today, the Milky Way gives birth to the equivalent of one Sun-sized star per year — a lull after a busy outburst.

Script by Damond Benningfield

Depending on your viewpoint, the Dog Days of summer are either in full swing or they’re just wrapping up. That’s because there’s no formal definition for the dates of the Dog Days. All we can say for sure is that this period got its name from the brightest star in the night sky.

Sirius is the leading light of Canis Major, the big dog. So it’s also known as the Dog Star.

The period of summer’s greatest heat came to be called the Dog Days in ancient Greece. The heat set in around the time Sirius made its first appearance in the dawn sky, in July. Since the star was so conspicuous, the Greeks associated the heat of summer with the Dog Star — so the period became known as the Dog Days.

Today, though, Sirius doesn’t make that first dawn appearance at mid-northern latitudes until August.

The change is thanks to an effect known as precession. Earth “wobbles” on its axis like a gyroscope that’s beginning to spin down. As it wobbles, the stars all shift position with respect to the seasons. So Sirius pops into view later in the summer than it did thousands of years ago.

Some sources define the Dog Days as the period from about July 3rd to August 11th. By that definition, the Dog Days end as Sirius first climbs into view. But others have pushed the Dog Days later to keep in line with Sirius’s dawn showing. So by those accounts, the Dog Days are still in full force.

Look for Sirius quite low in the east-southeast in early twilight — the star that brings the Dog Days.

Script by Damond Benningfield

The Canary Islands attract millions of visitors every year. They come for the warm climate, beautiful beaches, and rugged volcanic mountains. And many of them are attracted by another natural wonder: dark night skies. In fact, skywatchers account for several hundred thousand visitors a year. And thanks to laws that protect the view, the numbers are growing.

It’s not just tourists who admire the skies of the Canary Islands, which are off the west coast of Africa. Professional astronomers are there, too. The islands are home to two major observatories. The combination of clear skies and stable air give them some of the best viewing conditions on Earth.

Teide Observatory, on the island of Tenerife, got its first telescope in 1964. Today, it hosts an assortment of mostly small telescopes for nighttime viewing, and several more for watching the Sun.

The other observatory, Roque de los Muchachos, is 8,000 feet high on the island of La Palma. It includes the largest individual optical telescope mirror on Earth, the Grand Canary Telescope. The mirror is made of 36 segments, and it spans 34 feet.

Several other large telescopes share the site, which was dedicated in 1985. There are quite a few smaller telescopes, too, including some that automatically hunt for planets in other star systems.

These instruments help make the Canary Islands among the busiest of all skywatching sites — for the professionals, and everyone else, too.

Script by Damond Benningfield