StarDate

The residents of Wiseman, Alaska; Murmansk, Russia; and Tromsø, Norway, might be having a hard time getting a good night’s sleep. That’s because there’s no night to sleep through. All three towns are above the Arctic Circle, so they’re seeing the midnight Sun — a time when the Sun never sets.

There’s constant daylight because of the time of year. Today marks the summer solstice in the northern hemisphere. It’s the start of the summer season, and the longest day of the year north of the equator.

The Sun is farthest north for the year, and that’s the key. At the solstice, the north pole dips most directly toward the Sun. The Sun shines in a wide circle around the pole. Anything above about 66 degrees north latitude — the Arctic Circle — sees constant daylight.

How long the Sun stays above the horizon varies by latitude. Close to the Arctic Circle, it’s a few days or weeks. For those farther north, it’s longer. For the northernmost town in Alaska, for example, it’s almost three months. For the northernmost town in the world, on a Norwegian island, it’s more than four months. And for the north pole itself, it’s six months from sunrise to sunset.

Of course, it’s just the opposite in the southern hemisphere. So the team that spends the winter at the south pole is just half way through the long night. In essence, that makes it midnight — with three more months until the south pole sees its own “midnight Sun.”
 

Script by Damond Benningfield

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Every star in the night sky is moving around the galaxy in a hurry — hundreds of thousands of miles per hour. Yet the stars are so far away that it’s not easy to measure that motion.

Consider Spica, the bright star that hangs below the Moon as night falls.

Seen from here on Earth, the Moon is about half a degree wide. Now imagine cutting that small disk from top to bottom into more than 30 thousand equal slices. The amount that Spica moves across the sky in a year is the width of one slice. At Spica’s distance of 250 light-years, that works out to a speed of about 40,000 miles per hour relative to the Sun.

That sideways motion is known as proper motion. But it tells us only part of the story of Spica’s motion through space. The other part is its radial velocity — how fast it’s moving toward or away from us.

Proper motion is measured by watching how a star moves against the background of other stars. For radial velocity, though, astronomers have to take the spectrum of a star — they split its light into its individual wavelengths or colors. Those wavelengths are shifted by the star’s motion. They shift to redder wavelengths if the star is moving away from us, and bluer if it’s moving toward us.

Spica’s light is shifted toward the red. The size of that shift tells us that it’s moving away from us at about 2200 miles per hour — part of its overall motion through the galaxy.

Tomorrow: the midnight Sun.
 

Script by Damond Benningfield

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Most of the stars of the Milky Way were born here. But quite a few were born in other galaxies that were gobbled up by the Milky Way. Some of the remnants of those galaxies form long ribbons of stars, which makes them easier to find.

One of the most recent discoveries is known as the Jhelum Stream. It was discovered in 2018, by an observatory in Chile, and is named for a river in India. The stream runs across the constellation Grus, the crane, which is so far south that it’s not visible from most of the United States.

Jhelum is centered about 40,000 light-years from Earth. It’s in the Milky Way’s halo, which is far outside the galaxy’s disk. It’s hundreds of light-years long. And it appears to split into two ribbons. One is short and skinny, while the other is longer and wider. They move through space together, though, suggesting they have a common origin.

Astronomers are still trying to figure out just what that origin is. Most money says that both ribbons are remnants of a single small galaxy. It was a few million times the mass of the Sun. Although that sounds pretty heavy, it’s tiny compared to the mass of the Milky Way.

As the little galaxy approached the Milky Way, billions of years ago, it was captured by our galaxy’s powerful gravity. As it orbited the center of the galaxy, it was pulled apart. And today, its remaining stars form ribbons around the Milky Way — the remnants of a small but independent galaxy.

Script by Damond Benningfield

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Sagittarius, a sausage, a sequoia, and a Kraken appear to have something in common: All of them may be large galaxies that were absorbed by the Milky Way billions of years ago.

The Milky Way is a giant galaxy. It got that way by gobbling up other galaxies. Today, astronomers are tracking down the remnants of many of those galaxies — an endeavor known as galactic archaeology.

Many of the remnants are long ribbons of stars that loop through the Milky Way’s disk. Others are giant star clusters that loop far outside the disk. Stars in these ribbons and clusters are a bit different from the rest of the galaxy. They move differently, for example, and they have slightly different chemistry.

New tools help astronomers find those remnants. They’re using a space telescope, for example, to plot the motions of two billion stars.

The observations have revealed several possible big mergers when the Milky Way was young. The earliest was with a galaxy that’s been named Kraken, about 11 billion years ago. It may have been the most dramatic merger in the Milky Way’s history. Kraken appears to have been a few percent of the mass of the Milky Way, so it would have really stirred things up.

Over the following few billion years, the Milky Way swallowed several other big galaxies, including Sequoia, Gaia-Sausage, and Sagittarius — mergers that made the Milky Way even more impressive.

We’ll talk about a smaller merger tomorrow.
 

Script by Damond Benningfield

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A faint star in the serpent is getting closer — in more ways than one. For one thing, it’s moving toward us at about a third of a million miles per hour. And for another, every time astronomers recalculate how close the star will pass from the Sun, the distance goes down.

Today, Gliese 710 is about 64 light-years away. It’s in the constellation Serpens, in the east-southeast in early evening. But the star is fairly small and faint, so it’s far too dim to see with the eye alone.

Because it’s rushing toward the solar system, though, that won’t always be the case. In about 1.3 million years, it’ll be quite bright — as bright as Mars appeared last fall.

A couple of decades ago, astronomers calculated that the star would pass roughly one light-year from Sun and Earth. Since then, a space telescope has provided more accurate readings of its distance and its motion through space. With that, astronomers cut the minimum range to half a light-year, then a quarter. And a few months ago they cut it to just one-sixth of a light-year. By comparison, the closest star today is more than four light-years away.

With such a close approach, Gliese 710 should plow through the Oort Cloud — chunks of rock and ice that orbit far from the Sun. The star’s gravity could push some of them toward the inner solar system, where they could hit Earth — perhaps wiping out much of our planet’s life in that far-distant future.
 

Script by Damond Benningfield

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The Moon rattles a bit. The way it rattles can reveal details about how it’s put together, how it was born, and more. And scientists are pondering a network of instruments to listen in.

The Lunar Geophysical Network would consist of four robotic landers. Three of them would be on the hemisphere that always faces Earth, while the fourth would be on the farside. An orbiting satellite would relay its observations to Earth.

Each lander would carry a seismometer to listen for moonquakes. Astronauts left similar devices on the Moon decades ago, and they worked for years. They found that the Moon is much quieter than Earth, but it does rattle. A recent study of the quakes suggests that the Moon may be shrinking as it gets older and cooler.

Seismic waves from the quakes rattle through the Moon, bouncing off different layers. A network of listening devices could map those layers and learn what they’re made of.

Other instruments would measure the Moon’s temperature, and allow telescopes on Earth to precisely track “wobbles” in its rotation. Those details would help scientists piece together the Moon’s history.

To fly, the proposed mission will have to beat out other possible missions of exploration. If it does, it could head for the Moon in a few years.

For now, look for the Moon in the west at nightfall. The bright star Regulus, the “heart” of the lion, is close by, and stays close as they set in the wee hours of the morning.
 

Script by Damond Benningfield

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JFK: I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely to the Earth.

John F. Kennedy set that goal in a speech to Congress in May 1961. It led to the first lunar landing eight years later. Kennedy also asked Congress to support an even “spacier” goal: developing a nuclear rocket.

JFK: This gives promise of someday providing a means for even more-exciting and ambitious exploration of space — perhaps beyond the Moon — perhaps to the very end of the solar system itself.

NASA was already developing one of the tools it needed to build such a rocket: a nuclear reactor. The device produced its first sustained reactions 60 years ago today.

The reactor was at Plum Brook Station, near Sandusky, Ohio. Its original mission was to support development of nuclear-powered aircraft. It switched to rockets just days after Kennedy’s speech.

NASA eventually built and tested engines on the ground. And by the early 1970s, it was about ready to build an engine to test in space. But then its budget was slashed. Nuclear rockets were cancelled — and so was the Plum Brook reactor.

NASA considered plans for re-opening the facility, but nothing ever came of them. The last bits of the reactor site were cleared away in 2012. Today, though, NASA is poring over the old files as it once again considers building nuclear-powered rockets.

Script by Damond Benningfield

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A couple of prominent companions huddle near the crescent Moon this evening. The planet Mars is close below the Moon. And the Beehive star cluster is about the same distance to the left of the Moon. The view is best after the sky gets nice and dark; binoculars will enhance the view.

Through binoculars, you’ll be able to pick out all the notable features on the entire lunar disk — including the dark part, where it’s night. That’s because a gibbous Earth hangs in the lunar sky right now. It’s many times brighter than a full Moon, so it washes the Moon in reflected sunlight. So even to the eye alone, the dark portion of the Moon is easily visible — it has a pale ghostly appearance.

Binoculars don’t do much to enhance Mars. It looks like a moderately bright star either with or without them. Binoculars ought to show off its color a bit better, though.

But they will make a difference with the Beehive. Without the binoculars, it looks like a hazy patch of light, with perhaps a few individual stars visible to those with sharp eyes. But through binoculars it more closely resembles its name — it looks like a bunch of stellar bees buzzing through the night.

And as a bonus, there’s one bright object that’s visible in the fading twilight: Venus, the brilliant “evening star.” It’s well to the lower right of the Moon, and drops from sight by about the time the sky gets fully dark.

Tomorrow: a nuke for NASA.
 

Script by Damond Benningfield

Support McDonald Observatory

Most of the stars of the Milky Way were born here. But quite a few were born in other galaxies that were gobbled up by the Milky Way. Some of the remnants of those galaxies form long ribbons of stars, which makes them easier to find.

One of the most recent discoveries is known as the Jhelum Stream. It was discovered in 2018, by an observatory in Chile, and is named for a river in India. The stream runs across the constellation Grus, the crane, which is so far south that it’s not visible from most of the United States.

Jhelum is centered about 40,000 light-years from Earth. It’s in the Milky Way’s halo, which is far outside the galaxy’s disk. It’s hundreds of light-years long. And it appears to split into two ribbons. One is short and skinny, while the other is longer and wider. They move through space together, though, suggesting they have a common origin.

Astronomers are still trying to figure out just what that origin is. Most money says that both ribbons are remnants of a single small galaxy. It was a few million times the mass of the Sun. Although that sounds pretty heavy, it’s tiny compared to the mass of the Milky Way.

As the little galaxy approached the Milky Way, billions of years ago, it was captured by our galaxy’s powerful gravity. As it orbited the center of the galaxy, it was pulled apart. And today, its remaining stars form ribbons around the Milky Way — the remnants of a small but independent galaxy.

Script by Damond Benningfield

Support McDonald Observatory

Half of the constellations of the zodiac line up across the south as night falls this evening like charms on a bracelet. The lineup begins with Gemini, which is quite low in the west-northwest. Both the crescent Moon and Venus, the “evening star,” are within its borders. Cancer is to the upper left of the twins. Then the lineup arcs across the south to include Leo, Virgo, Libra, and finally Scorpius, low in the southeast.

In ancient times, skywatchers assigned special meaning to these constellations because they’re on the ecliptic — the Sun’s path across the sky — so the Sun moves across them each year.

Another constellation also lies along this path, but it’s not a member of the zodiac. Ophiuchus is to the upper left of Scorpius at nightfall. Its brightest members form the faint, wide-spread outline of an old coffee urn.

Ophiuchus isn’t an official member of the zodiac because the Sun doesn’t cross the urn itself. Instead, it just nips the southern edge of the modern version of the constellation, which includes a wide area around the classical outline.

All of the modern constellations have well-defined borders. That turns the sky into a giant quilt with 88 patches. They’re of many different sizes and shapes. Some are simple rectangles, but most have odd outlines to hold the classical star pictures. In fact, the border of Ophiuchus consists of 38 segments — an oddly shaped patch that just intersects the ecliptic.

Script by Damond Benningfield

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