StarDate

If you look at a map of just about any state, you’ll see a few major cities, a few more mid-sized ones, and a whole bunch of small towns. And the same thing happens if you look at a map of our galactic neighborhood. There are a couple of giant galaxies, a few mid-sized ones, and a whole bunch of little ones. They’re all bound to each other by their mutual gravitational pull.

This collection of galaxies is known as the Local Group. It spans about 10 million light-years, and contains a hundred or so known members, with more added every year.

The “big cities” of the Local Group are our home galaxy, the Milky Way, and the Andromeda Galaxy, M31. Each is a disk that’s at least a hundred thousand light-years wide, and contains hundreds of billions of stars.

After these two behemoths, the list includes a few mid-size members. The largest is the Triangulum Galaxy. It’s about half the diameter of M31 and the Milky Way, but only a few percent as massive.

A couple of mid-size galaxies are satellites of M31. One is a spiral, while the other is a small elliptical galaxy, which looks like a fuzzy rugby ball. And two more members of the mid-size brigade orbit the Milky Way — the Large and Small Magellanic Clouds.

Most of the other members of the Local Group are tiny — far smaller, and with far fewer stars, than the larger galaxies. Many are satellites of the Milky Way, and we’ll have more about them tomorrow.
 

Script by Damond Benningfield

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Under the current western calendar, tomorrow is the 16th day of the month. But under another system, it’s the first day of the month — a month based completely on the phases of the Moon.

The concept of a “month” came from that cycle of phases. The cycle lasts an average of 29 days, 12 hours, 44 minutes, and 3 seconds — roughly the length of a calendar month. In fact, the word “month” means “Moon.” But in the modern calendar, each month is a whole number of days. So the lunar phases drift — they don’t occur on the same day of the month.

The true lunar month is called a lunation. It begins and ends at new Moon — when the Moon passes between Earth and the Sun, and is hidden in the Sun’s glare. And the Moon will be new in the wee hours of tomorrow morning for those of us in the United States, starting a new lunar month.

Lunations aren’t all quite the same length. The Moon’s orbit around Earth is lopsided, and the phases don’t stay in sync with the Moon’s changing distance. So the length of a lunation can vary by about seven hours in either direction. The Moon is at its farthest from Earth tonight, so this lunation will be about three and a half hours longer than the average. And under a numbering system developed in the 1930s, it’s lunation number 1245.

The Moon will return to view on Thursday, as a thin crescent quite low in the west after sunset — the first appearance of a “new” lunar month.
 

Script by Damond Benningfield

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At first glance, Gliese 913 is a pretty average star system. It’s a binary — two stars that travel through space together, bound by gravity. The main star is smaller, fainter, and cooler than the Sun. And its companion is even more modest.

But when astronomers listened, Gliese 913 got a lot more interesting. A recent analysis of its radio waves found two signals that could be of intelligent origin. But a follow-up session heard nothing. So no one knows whether the signals came from another civilization, or from here on Earth.

The study was conducted by Breakthrough Listen, a major search for extraterrestrial intelligence. Its astronomers are using large radio telescopes to scan millions of stars.

They recently used a technique known as machine learning to analyze observations of more than 800 of those stars. The computers found about three million interesting signals. But a detailed look showed that almost all of them came from Earth — from satellites, cell phones, or other sources.

But eight signals, from five stars, stayed interesting — including the two from Gliese 913. So astronomers listened to the stars again. This time, they didn’t hear anything unusual. So they can’t rule out any possible origin — natural, terrestrial, or extra-terrestrial.

Gliese 913 is in the constellation Andromeda, which is in the northeast at nightfall. But you need a telescope to see this intriguing star system.
 

Script by Damond Benningfield

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After commanding the western sky as the “evening star” for most of the year, the planet Venus is making a move. It’s disappeared from view as it crosses between Earth and the Sun. It’ll return to view in a few days — this time as the morning star.

Venus is closer to the Sun than Earth is — like being on the inside lane of a race track. So it overtakes Earth every 584 days — about 19 and a half months. That point is called inferior conjunction, and it’s where Venus is today.

The planet is also closest to Earth — about 27 million miles away. And that means it moves across our sky in a hurry — the reason it’ll stay out of sight for only a few days, hidden in the Sun’s glare.

When Venus returns to view, it’ll be a thin crescent — like a crescent Moon. The crescent will grow thicker in a hurry, though, making the planet look brighter. But Venus will be moving away from us, which makes it fainter. When you combine those factors, Venus will reach its peak brilliance on September 18th. It’ll slowly fade through the rest of the year, but not by a whole lot. It will remain the brightest object in the night sky other than the Moon.

Depending on your location, Venus should climb into view, quite low in the east, during the dawn twilight in the next week or two. It’ll move farther from the Sun day by day after that, taking command of the morning sky — as the brilliant “morning star.”
 

Script by Damond Benningfield

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Thousands of students around the country and around the world are getting ready to go to the Moon. They’re developing cracker-sized instruments to land on the lunar surface. Several hundred of these instruments will share a ride to the Moon. They’ll operate independently and in groups, addressing several topics in lunar science.

The project is known as GLEE — Great Lunar Expedition for Everyone. It’s an outreach effort of Artemis — NASA’s project to return astronauts to the Moon. It’s open to high school and college students around the world.

Each team is customizing a computer chip that’s a couple of inches on each side. The solar-powered chips include sensors to measure the temperature, the Moon’s magnetic field, and other parameters. Several hundred of them will hitch a ride on a lunar lander. They’ll then be fired out of a small canister, and spread across several hundred square yards. The students will analyze the readings from their chips, which will be made available to everyone.

GLEE is still looking for a landing mission to carry the chips — the work of thousands of students from around the world.

Their target — the Moon — will be in the southeast at first light tomorrow, near the center of Gemini. The constellation’s “twins” will perch close by. Pollux, the brighter of the twins, will be to the lower left of the Moon, with Castor almost due left.

Tomorrow: Another light in the dawn sky.
 

Script by Damond Benningfield

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One of the year’s best meteor showers will be at its best the next couple of nights. Under dark skies, you should see up to a few dozen “shooting stars” per hour — especially tomorrow night, at the shower’s peak.

The Perseid meteor shower gets its name from the constellation Perseus. If you track the meteors across the sky, that’s where their paths converge. But the meteors can appear anywhere, so you don’t need to look toward the constellation to see them. The view is better when Perseus climbs higher into the sky, though, in the wee hours of the morning.

While the shower gets its name from Perseus, it gets its meteors from Comet Swift-Tuttle — a mountain-sized ball of ice and rock that orbits the Sun once every 133 years or so. As the comet gets close to the Sun, it warms up. Some of its ice vaporizes, and small bits of ice, rock, and dirt fly out into space.

These particles spread out along the comet’s orbit. Earth zips through the trail of comet dust every summer. Bits of debris ram into the atmosphere at more than a hundred thousand miles per hour. They vaporize, forming the glowing streaks of light known as meteors.

To watch the Perseids, find a dark but safe viewing site, well away from the glow of city lights. The best view comes in the hours before dawn on Sunday. The Moon will be a thin crescent, so it won’t spoil the show put on by the Perseids — one of the year’s best meteor showers.
 

Script by Damond Benningfield

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Messier 31 — the Andromeda Galaxy — is the hub of an empire. It’s orbited by at least 40 smaller galaxies, all held in place by M31’s powerful gravity.

One of its largest satellite galaxies was discovered 250 years ago today, by French astronomer Charles Messier.

Messier’s main interest was comets — hazy balls of ice and rock. At the time, finding comets brought fame and fortune. But the sky is filled with fuzzy objects that resemble comets. Messier compiled a catalog of more than a hundred of those objects so that he and others could ignore them.

While observing Messier 31 — the 31st object in his catalog — Messier found a small, bright blob next to it. He took some notes and drew a picture of it, but he didn’t list it in his catalog. Two centuries later, though, astronomers decided to add it. It became Messier 110 — the final object on Messier’s list.

M110 is a dwarf elliptical galaxy. It’s less than one-sixth the diameter of our home galaxy, the Milky Way, and only a few percent as massive. It doesn’t have spiral arms or other obvious structures — just a bright center with stars fading away toward the edges.

M31 is low in the northeast not long after darkness falls, and passes high overhead not long before dawn. Under dark skies, it’s just visible to the unaided eye as a hazy smudge of light. M110 is quite close to it, but it’s too faint to see without a telescope.
 

Script by Damond Benningfield

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Five of the stars that make up the Big Dipper are like a great report card: straight “A”s. All five stars are members of class A — stars that are bigger, hotter, and brighter than the Sun.

The dipper is in the northwest at nightfall, with its bowl below the handle. The stars at the tip of the handle and the lip of the bowl are outliers — they line up in the same direction as the rest of the dipper, but otherwise they’re unrelated.

But the five stars in the middle have a lot in common. They were born together, from the same giant cloud of gas and dust. Their nursery has long since broken up, but they still hang out together — they move through the galaxy at about the same speed, and in the same direction. They’re all roughly 80 light-years from Earth. And all five stars are class A. That puts them in the top one percent or so of all the stars in the galaxy.

The stars range from about one and a half to three times the mass of the Sun, and from about 15 to a hundred times the Sun’s brightness. And as a reflection of their common birth, they’re all just a fraction of the Sun’s age — a few hundred million years old.

From the second star in the handle to the lower corner of the bowl, the class-A stars are Mizar, Alioth, Megrez, Phecda, and Merak. Megrez, which connects the handle to the bowl, is by far the smallest and faintest. But it’s still impressive — one of the “straight-A” stars of the Big Dipper.
 

Script by Damond Benningfield

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The Moon, the planet Jupiter, and the star Aldebaran form a long, skinny triangle in the eastern sky at first light tomorrow. Jupiter looks like a brilliant star, to the upper right of the Moon. Fainter Aldebaran is the same distance to the lower left of the Moon.

The three bodies are at different distances from Earth. That means we see them as they looked at different times: a little more than a second ago for the Moon, 40 minutes for Jupiter, and 65 years for Aldebaran.

That’s how long it takes the light to reach Earth from our perspective. But from the perspective of the light itself, there’s no difference at all.

Light travels in “packets,” known as photons. They move at the speed of light — 186,000 miles per second. Albert Einstein’s theory of special relativity says that, as measured by an outside observer, time doesn’t pass for anything moving at lightspeed. From a photon’s perspective, it’s born, it races through space, and it dies in the same instant.

Relativity says that clocks tick at different rates for objects that are moving at different velocities. But an object needs to move at a big fraction of the speed of light for the effect to become noticeable. And at the speed of light, the clock wouldn’t tick at all.

So, on our clocks, a photon that leaves Aldebaran tonight will travel 65 years to reach Earth. But for the photon itself, when it hits the eye of a skywatcher in 2088, no time will have passed at all.
 

Script by Damond Benningfield

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The Moon and Jupiter form a beautiful pairing late tonight. Jupiter looks like a brilliant star. It’s quite close below the Moon as they climb into good view, after midnight.

Jupiter is the largest and heaviest planet in the solar system — 11 times wider and more than 300 times as massive as Earth. In fact, its core alone is more than 10 times Earth’s mass. But it’s not in the form that scientists had expected.

Until a few years ago, most ideas of how Jupiter is put together said it had a solid core made of rock and metal. But the Juno spacecraft has found otherwise. It’s measured Jupiter’s magnetic and gravitational fields from orbit. Those observations have given scientists a much better picture of how Jupiter is put together.

They show that the core is “diffuse,” like a scoop of ice cream left on the kitchen counter for a few minutes. The core melts into the surrounding layers of material.

One possible explanation is that a planet about 10 times the mass of Earth slammed into Jupiter when it was quite young. It would have smashed the core like a bowling ball hitting a set of pins, sending them flying in all directions. The core then stayed scattered and mushy.

But a recent study suggests the core would have pulled itself back together within hours of the impact, no matter what the speed or angle of the collision. So scientists may still have some work left to explain Jupiter’s “melty” center.
 

Script by Damond Benningfield

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