We can think of our location in the universe along the lines of an address. The street would be planet Earth, the county would be the solar system, and the country would be the Milky Way Galaxy. The state would be the Orion Arm – a ribbon of stars that wraps part of the way around the galaxy.

The Milky Way is a disk that’s about a hundred thousand light-years wide. It has a long “bar” of stars in its middle. Spiral arms extend from the ends of the bar and wrap all the way around the galaxy. They make the Milky Way look like a pinwheel spinning through the void.

The arms don’t contain more stars than the darker regions between them. Instead, the arms are like waves on the ocean. As a wave washes through the galaxy, it squeezes clouds of gas and dust, giving birth to new stars. Many of those stars are big, hot, and bright. So they make the spiral arms look bright and blue. But such stars die quickly, so the wave of brightening doesn’t last.

A few shorter arms fill in between the major ones. And the Orion Arm fits into that category. It’s about 3500 light-years wide and 20,000 light-years long. At our distance from the center of the Milky Way, the arm wraps only about a quarter of the way around the galaxy.

The arm is named for Orion because of the arm’s location in the sky. The stars of Orion are among its most prominent members. But the arm includes almost all of the stars that are visible to the unaided eye.

Script by Damond Benningfield

Individually, the stars of Orion’s Belt aren’t that impressive. We pay attention to them mainly because they’re tightly grouped – a short line that’s easy to pick out. But their appearance is wildly deceiving. The stars are among the most impressive in the entire galaxy. They have to be impressive for us to see them at all, because they’re about 1200 light-years away.

Orion’s Belt is low in the east-southeast at nightfall. It extends almost straight up from the horizon. From bottom to top, its stars are Alnitak, Alnilam, and Mintaka.

Alnitak is a system of three stars. The main star is a monster – more than 30 times the mass of the Sun. Its surface is tens of thousands of degrees hotter than the Sun, so Alnitak shines blue-white. And it’s about 250,000 times brighter than the Sun.

Alnilam is a single star. And it appears to be an even bigger monster: perhaps 40 times the Sun’s mass, and 400,000 times its brightness.

Mintaka is the most complex member of the belt – a family of five stars. All of them dwarf the Sun in every possible way.

The stars in these systems are only a few million years old. But they’re so massive that they’re already nearing the end. Some of them will blast themselves to bits, leaving behind only a dead core – a neutron star or a black hole. But some might be too heavy for that. The entire star might collapse to form a black hole – impressive fates for impressive stars.

Script by Damond Benningfield

To the ancient Maya, the Orion Nebula was the fire of creation. The bright stars around it were the hearthstones that held the fire.

And they were on the right track. The nebula is a giant stellar nursery. It’s given birth to thousands of stars, with more still taking shape. And the “hearthstone” stars are related to the fire. The whole group is part of a massive complex of young stars, plus lingering clouds of gas and dust.

It’s known as the Orion O-B-1 association. The letters refer to the group’s many class O and B stars, which are much bigger and brighter than the Sun. Such stars live for millions of years, versus billions of years for the Sun.

The association consists of four big clumps. One of them is north of Orion’s Belt. Another includes the belt. The others are south of the belt, with the Orion Nebula at the end of the chain.

The stars in the northern group are the oldest. They’ve blown away the remaining gas, so no new stars are being born there. The Orion Nebula is in the youngest group. Some of its stars are less than a million years old. The stars of the belt are between those ages; more about the belt tomorrow.

The association also includes thousands of smaller, fainter stars. But the O and B stars are the real standouts.

Orion is in the east and southeast at nightfall. Its belt extends upward from the horizon. The Orion Nebula looks like a fuzzy star to the right of the belt – a fiery hearth of creation.

Script by Damond Benningfield

The Sun is a middle-aged star. It’s been shining for about four and a half billion years, and it’s expected to keep on shining for billions of years more. But some stars that are only a fraction of the Sun’s age are well into the final stages of life. These stars are much more massive than the Sun, and it’s a star’s mass that determines its timeline.

An example is Arneb, the leading light of Lepus, the rabbit. The constellation stands below the feet of Orion the hunter. The rabbit is in the southeast at nightfall.

Arneb is only about 13 million years old, yet it’s well into old age. In fact, it’s likely to expire within the next million years or so. That’s because Arneb is about 14 times the mass of the Sun. Its powerful gravity squeezes its core tightly. That heats the core, revving up the nuclear reactions that power the star.

The extreme radiation from the core pushes on the surrounding layers of gas, making the star puff up – Arneb is more than a hundred times the diameter of the Sun. If it took the Sun’s place in our own solar system, it would gobble up the planet Mercury and come close to Venus, the second planet from the Sun. And the star may get even bigger in the future.

Eventually, Arneb will no longer be able to generate nuclear reactions in its core. The core will collapse, and the star’s outer layers will blast into space – bringing the short, brilliant life of Arneb to a fiery end.

Script by Damond Benningfield

Venus and Saturn look a bit jaundiced. But they achieve their yellowish color in different ways.

Venus is the second planet from the Sun. It’s about the same size as Earth. It’s topped by a perpetual blanket of clouds. The clouds are made of sulfuric acid, so they give the planet a bright yellow-white color.

Saturn is covered by clouds as well, but they consist of water vapor, ammonia, and hydrocarbons. The clouds are stretched into globe-encircling bands by Saturn’s high-speed rotation. Each band reveals a different layer of the atmosphere. The layers have slightly different mixtures of compounds, so they have slightly different colors – shades of white, yellow, and tan. Seen from afar, they give Saturn an overall hazy yellow-brown appearance.

Other than their color, there’s not much resemblance between the two planets. Venus is a rocky world like Earth, and its surface is blazing hot. Saturn is the second-largest planet in the solar system – a big ball of gas with a small solid core. And it’s encircled by bright, beautiful rings.

Venus and Saturn are in the southwest at nightfall. Venus is the brilliant “evening star” – the brightest point of light in the night sky. Tonight, Saturn stands quite close to the left of Venus. It’s only about half a percent as bright as Venus, but still easy to find. Venus will slide above Saturn on the following evenings. But they’ll stay close together for several nights.

Script by Damond Benningfield

Astronomy is just like any other human endeavor. It can stir up controversy, hurt feelings – and feuds. One of its most famous feuds involved some of the giants of science – Isaac Newton, who formulated laws of gravity and motion, and Edmond Halley, the namesake of Comet Halley.

Newton and Halley were colleagues of John Flamsteed, Britain’s first astronomer royal and the founder of the Royal Observatory.

Flamsteed had compiled a catalog of more than 2,000 stars. It included precise positions of the stars along with other details. He showed the catalog to Halley and Newton. They were impressed, and they wanted to use its findings in their own research. So they encouraged Flamsteed to publish the catalog.

But Flamsteed refused. Halley and Newton then went behind his back. With the support of the Royal Society, they published 400 copes of it, in 1712.

Flamsteed was furious. He bought up all the copies he could find – and burned them. And he waged a feud against Halley and Newton for the rest of his life. The catalog wasn’t officially published until six years after his death.

The first version included new designations for the stars – a number followed by the name of the constellation. One of those designations is 32 Leonis. The star is better known as Regulus – the brightest star of Leo. And it’s especially easy to spot tonight. It’s above the Moon as they climb into good view, around 9 or 9:30.

Script by Damond Benningfield

The moons of Mars are puny little things – small, oddly shaped chunks of rock that orbit close to the planet. But Phobos and Deimos could be indirect descendants of a much larger moon – one that was up to a third as massive as Earth’s moon. And a recent study says that early moon could explain something about Mars itself.

Mars is oddly shaped. It has two big “bulges” that line up roughly opposite each other on the globe. One of them is the highest landscape on the planet, and it’s crowned by giant volcanoes.

The study says the shape could be the result of the gravitational pull of a big moon. Researchers named the moon Nerio for a goddess who was an early ally of Mars.

Nerio would have stayed over the same spot on Mars. If it was there early on, when the Martian surface was molten, then its gravity could have created the bulges. Mars then cooled quickly, locking the bulges in place.

The moon wouldn’t have lasted long. It could have been kicked away by a close encounter between Mars and another large body, or shattered in a giant impact. A chunk of the moon could have survived. Then it, too, was shattered, leaving Mars with only Phobos and Deimos.

The scenario raises a lot of questions. So it’ll need much more study before scientists can confirm that Mars once had a big moon.

Mars is in view all night. It looks like a bright orange star. It’s in the east-northeast at nightfall, and climbs high across the south during the night.

Script by Damond Benningfield

As far as we can tell, the surface of Mars is lifeless. But there are hints that microscopic organisms might live below the surface. One of the strongest hints is occasional puffs of methane gas detected by the Curiosity rover, which has been scooting around Mars for more than a decade.

On Earth, methane is mainly produced by life. It can come from volcanoes and from chemical reactions in the rocks. But there are no active volcanoes on Mars, and chemical reactions seem unlikely to produce big puffs of the gas.

Curiosity’s instruments have detected several outbursts of methane. They’ve all happened at night, and most have taken place in late spring or early summer, when the ground is warming up. A few of the outbursts have been especially potent.

The methane could come from microbes far below the surface, perhaps living in small pockets of water. Such organisms have been found on Earth. They combine hydrogen and carbon molecules to make methane. The methane could be trapped in bubbles that are freed when the ground gets warmer, or when the rover cracks the surface above them – releasing methane into the atmosphere.

That’s only one possible explanation for the methane. But it provides a hint that Mars could be inhabited by “gassy” microbes.

Mars is shining at its best this month. It looks like a brilliant orange star. It’s low in the eastern sky at nightfall, and remains in view all night. More about Mars tomorrow.

Script by Damond Benningfield

Mars is at its closest and brightest this week. And it’s in view all night – almost. Tonight, it’ll vanish for a while for skywatchers from western Africa to the American west coast – hidden behind the Moon.

The disappearing act is an occultation. The word comes from Latin, and means “to hide” or “to cover up.” And the occultation is the result of the geometry of the solar system.

Mars and the other planets all stay close to the Sun’s path across the sky, the ecliptic. So does the Moon. But their orbits are tilted a little bit with respect to the ecliptic. So most months, the Moon and Mars pass close to each other, but they don’t overlap. The arrangement has to be just right for the Moon to occult the Red Planet.

Occultations come in clumps, when Mars and the Moon are in phase with each other and with the ecliptic. They’re in one of those phases now. There was an occultation last month, and there’s another one next month.

Tonight’s occultation takes place during the evening hours. The Moon will drop toward Mars, which looks like a bright star, then pass in front of it. The entire occultation will last about four hours. But from any given location, Mars will be out of sight for no more than about one hour. The occultation will be visible across the contiguous United States, eastern Canada, and most of Mexico. Mars and the Moon will stay close together for the rest of the night.

More about Mars tomorrow.

Script by Damond Benningfield

Mars is especially fetching this month. That’s because it lines up opposite the Sun on Thursday. It rises around sunset and remains in view all night. It’s also brightest for the year – it looks like a brilliant orange star. It shines as intensely as Sirius, the brightest true star in the night sky.

Mars looks bright because it passes closest to Earth for the entire year today. It’s about 60 million miles away. That’s not an especially close opposition; at its best, Mars can approach to within about 35 million miles.

But Mars’s orbit is lopsided, so the planet’s distance from both the Sun and Earth varies by tens of millions of miles. This time, we’re catching Mars near its farthest point, making this one of the planet’s fainter oppositions.

But the view is enhanced by a couple of nearby stars: Pollux and Castor, the twins of Gemini. Pollux is closer to Mars, and the two of them will snuggle even more tightly over the coming nights. At their closest, they’ll be only about two degrees apart – the width of your finger held at arm’s length. Mars and Pollux have a similar color, but Pollux is only about one-tenth as bright as Mars.

The Moon will heighten the drama tomorrow night. It’ll cross in front of Mars, blocking it from view. The disappearing act will be visible from the Lower 48 states. At most, Mars will remain out of view for a little more than an hour. We’ll have more about that tomorrow.

Script by Damond Benningfield