The Milky Way is packed with star clusters – thousands of them. They contain anywhere from a few dozen stars to more than a million. And the most impressive of them all is right in the middle – it surrounds the supermassive black hole at the heart of the galaxy. The Nuclear Star Cluster contains up to 10 million stars. They extend a couple of dozen light-years from the black hole in every direction. But most of them are packed in close. If our part of the galaxy were that densely settled, we’d have a million stars closer to us than our current closest neighbor, Alpha Centauri. So any planets in the cluster would never see a dark night. Most of the stars in the cluster formed about 10 billion years ago, when the galaxy was young. But there was another wave of starbirth about three billion years ago, and a smaller one just a hundred million years ago. Each wave might have been triggered when the Milky Way swallowed a smaller galaxy. As the galaxies merged, clouds of gas and dust settled in the middle, around the Milky Way’s black hole. That gave birth to new stars – populating the galaxy’s most impressive cluster. The cluster is in Sagittarius, which is due south at nightfall. The constellation looks like a teapot. The center of the galaxy is in the “steam” rising from the spout. But giant clouds of dust absorb the light from the galaxy’s heart, so it takes special instruments to see the cluster. Script by Damond Benningfield

Guillaume Le Gentil spent more than 11 years away from his native France just to witness two brief astronomical events. Along the way, he had to survive war, a hurricane, disease, and grumpy officials. When he got home, he’d lost his job and been declared dead. But the real hardship? He missed both events. Le Gentil was born 300 years ago this week. He studied theology, but decided on astronomy as a career. He became a member of the Royal Academy of Science at age 28. Le Gentil and other astronomers hoped to measure a 1761 transit of Venus across the Sun from many locations on Earth. The details would reveal the Sun’s distance – the basic “yardstick” for the entire solar system. Le Gentil planned to watch from India. He headed out in March of 1760. War with England complicated the trip, and his ship was blown off course. On the day of the transit he was still at sea, where it was impossible to make observations. The next transit was just eight years away, so Le Gentil decided to hang around. He planned to watch from the Philippines. But he got a chilly reception, so he returned to India. He set up an observatory and waited. But the day of the transit was cloudy – until shortly after it was over. Heartbroken, Le Gentil headed home. It took two hard years to get there – only to encounter even more problems. But he worked things out, and published two volumes about his travels in the name of science. Script by Damond Benningfield

If you’d like to travel into the future – even the far-distant future – you don’t need a time machine. Instead, a starship will do just fine. Fire up the engines, head into space, and keep your foot on the gas. The laws of physics seem to make it impossible – or nearly so – to travel through time in anything like the modern concept of a time machine – something that allows you to move through the centuries at will. Yet those same laws make it possible to zoom into the future. The concept is known as time dilation. As you travel faster, your clock ticks more slowly compared to the clocks of those you left behind. It’s been proven by putting atomic clocks in airplanes and aboard GPS satellites. In fact, if GPS clocks weren’t adjusted to account for it, the entire system would fail. At the speed of a satellite, the difference is tiny – a few millionths of a second per day. As speed increases, though, the effect becomes more significant. If you could travel at 90 percent of the speed of light for one year as measured by the clock on your ship, more than two-and-a-quarter years would pass back on Earth. At 99 percent of lightspeed, it’s more than seven years per ship year. And at 99.99 percent, the ratio is 70 Earth years per ship year. Of course, there is the problem of finding a fast starship to carry you. But so far, that’s the only known way to beat Time – and travel into the future. Script by Damond Benningfield

Based on the number of books, movies, and TV shows about it, you might assume that traveling through time is almost as easy as ambling through the park on a sunny day: Just build a TARDIS or soup up your Delorean, and off you go. Alas, the arrow of time moves in only one direction. It allows you to travel into the future, but roadblocks seem to prevent any method that scientists can envision for traveling in the other direction. Wormholes, for example, are theoretical “tunnels” through space and time. They seem to allow travel to other times – past or future. But there’s a problem: The wormhole may collapse as soon as anything enters it – a person, a spaceship, or even a radio beam. Another possibility for traveling into the past is moving really fast. Albert Einstein’s theories of relativity suggest that anything moving faster than light might move backward in time. But any physical object moving at lightspeed would become infinitely massive. That means you’d need an infinite amount of energy just to reach lightspeed – and even more to go faster. A few decades ago, Stephen Hawking suggested that the universe doesn’t like time travel. He wrote that the laws of physics may stop anyone from ever building a time machine – keeping the past safe from its own future. Even so, physics provides some tricks that allow travel to the future, and we’ll have more about that tomorrow. Script by Damond Benningfield

If a cosmic giant sat on a big, gassy planet, it would look a lot like Saturn, the second-largest planet in the solar system. It’s 10 percent wider through its equator than through the poles. But Saturn flattened itself – a result of its low density and fast rotation. Saturn consists of a series of layers. Its core is a dense ball of metal and rock. Around that is a layer of hydrogen that’s squeezed so tightly that it forms a metal. Around that is a layer of liquid hydrogen – the lightest and simplest chemical element. And the planet is topped by an atmosphere that contains methane, ammonia, water, and other compounds. Despite its great size, Saturn spins once every 10.7 hours. That pushes material outward, making the planet fatter through the equator. The combination of its composition and rotation makes Saturn especially light – it’s less dense than water. Saturn doesn’t have a solid surface. But scientists have defined a “surface” as the depth in its atmosphere where the pressure equals the surface pressure on Earth. At that level, Saturn’s gravity is only a bit stronger than Earth’s gravity. So if you were floating at that altitude, you’d feel like you’d added a few pounds. And because of Saturn’s flattened shape, you’d feel heavier at the poles than the equator. Look for Saturn near the Moon tonight. It looks like a bright star to the right of the Moon in early evening, and farther below the Moon at dawn. Script by Damond Benningfield

Building the planets of the solar system was like building a city – it didn’t happen all at once. Instead, it probably took a hundred million years or more to complete the construction project. The first to be completed were Jupiter and Saturn, the Sun’s largest planets. They came together in the prime real estate for planet building – the region with the most raw materials. Closer to the Sun, it was so hot that ices were vaporized and blown away. Farther from the Sun, the material thinned out. But at the distance of Jupiter and Saturn, the balance was just right. The two giants took shape in a hurry. Small grains of ice and rock stuck together to make pebbles, then baseball-sized chunks, then boulders, and so on. That quickly built massive cores, which then swept up huge amounts of leftover hydrogen and helium gas. So within just a few million years, Jupiter and Saturn have grown to monstrous proportions. Uranus and Neptune took shape a little later – within tens of millions of years. Earth and the other rocky inner planets took a bit longer – at least a hundred million years. So the biggest planets of the solar system are also the oldest – dating to shortly after the birth of the Sun. Saturn stands close to the Moon the next couple of nights. The planet looks like a bright star. It’s to the lower left of the Moon as darkness falls tonight, and about the same distance to the right of the Moon tomorrow night. Script by Damond Benningfield

The 41st episode of a celestial series plays out tomorrow: a total lunar eclipse. It’ll be visible around much of the world – but not the Americas. Every eclipse belongs to a series, called a Saros. The eclipses in a Saros are separated by 18 years plus 11 and a third days. If we could watch all the eclipses in the cycle play out, we’d see the Moon pass through Earth’s shadow from top to bottom or bottom to top. So the Moon barely dips its toe in the shadow at the beginning and end of the sequence. But it’s fully immersed during the middle of the cycle, creating total eclipses. And because of that extra third of a day in the cycle, each eclipse occurs a third of the way around the world from the previous one. This eclipse is part of Saros 128. The cycle began in 1304 and will end in 2566 – 71 eclipses in all. Most of Asia and Australia will see this entire eclipse, from beginning to end. And most of the rest of the world will see at least part of it. Totality – when the Moon is completely immersed in the shadow – will last for an hour and 22 minutes. But the eclipse occurs during the middle of the day for those of us in the United States, so we won’t see any of it. What we will see the next couple of nights, though, is a beautiful full Moon – the Fruit Moon or Green Corn Moon – completely free of Earth’s dark shadow. Script by Damond Benningfield

The Moon will briefly cover up the tail of the sea-goat tonight – Deneb Algedi, the brightest star of Capricornus. The sequence will be visible across much of the United States. This vanishing act is an occultation – a type of eclipse in which one object completely covers another. But eclipses are nothing new for Deneb Algedi. Not only does it periodically get covered up by the Moon, but it stages its own eclipses – two of them every day. What we see as Deneb Algedi is a binary – two stars in a tight orbit around each other. The main star in the system is about twice as big and heavy as the Sun, and much brighter. Its companion is a little smaller and fainter than the Sun. We’re looking at the system edge-on, so the stars pass in front of each other – creating eclipses. When the fainter star crosses in front of the brighter one, the system’s overall brightness drops by about 20 percent – enough for a skilled skywatcher to notice. But when the brighter star eclipses the fainter one, the dip is much smaller, so it’s detectable mainly with instruments. The stars orbit each other once a day. That means we see two eclipses per day – just 12 hours apart. Deneb Algedi isn’t especially bright, so it’s hard to see through the bright moonlight. But binoculars will help you pick it out. From much of the western U.S., the Moon will just miss the eclipse-happy tail of the sea-goat. Script by Damond Benningfield

The planet Jupiter will slide past one of the brighter stars of Gemini the next few mornings. At their closest, they’ll be separated by just a fraction of a degree. The star is Wasat – from an Arabic phrase that means “the middle.” But the middle of what has been lost over the centuries. The star also is known as Delta Geminorum – its Bayer designation. The system was devised in the early 17th century by German astronomer Johann Bayer. He named all of the stars in the constellations that were visible from the northern hemisphere. Each star was given a Greek letter followed by the constellation name. If he ran out of letters, he switched to the Latin alphabet. In most constellations, Bayer named the stars in the order of their brightness. The brightest was alpha, the next-brightest was beta, and so on. Sometimes, he ranked the stars on their location or some other system. And he named the stars based on how they looked to the naked eye, so the rankings were completely subjective. So even though delta is the fourth letter in the Greek alphabet, Delta Geminorum is only the eighth-brightest star in Gemini. Jupiter and Wasat are well up in the east at dawn. Jupiter looks like a brilliant star, far to the upper right of even-brighter Venus. Wasat will stand below Jupiter tomorrow. Jupiter will drop past it over the following couple of days, so they’ll be at their closest on Saturday and Sunday. Script by Damond Benningfield

Water is the key ingredient for life on Earth. And as far as we know, it’s a key ingredient for life everywhere else in the universe as well. That shouldn’t be a problem, though, because there’s plenty of water to go around. Water is common in part because it’s made of two of the three most common elements in the universe – hydrogen and oxygen. They come together in the cold of deep space to make grains of ice. Some of those grains are found in the clouds of gas and dust that give birth to new stars and planets. Others form inside those clouds. In recent years, astronomers have found evidence of water in other star systems, and even in other galaxies. They’ve found grains of ice in the disks of material around newborn stars. They’ve seen giant belts of comets, which contain a lot of ice. They’ve discovered water vapor in the atmospheres of a few planets. And they’ve even found evidence that some planets could be covered in oceans of liquid water. One example is TOI 1452 b, which orbits a star that’s much smaller and fainter than the Sun. The planet itself is bigger and heavier than Earth. Given its details and its distance from the star, scientists say it could have a deep global ocean – a possible home for life. TOI 1452 is about a hundred light-years away, in Draco. The dragon twists high across the north at nightfall. But the star is much too faint to see without a telescope. Script by Damond Benningfield