StarDate Online - Your guide to the universe

  • Summer Triangle

    The Summer Triangle is in good view at nightfall. Its brightest point is Vega, in Lyra, the harp, which is high in the east-northeast. The faintest point, Deneb, is well to the lower left of Vega, with Altair farther to the lower right of Vega.

  • Summer Triangle

    The sky offers plenty of calendar markers to help note the turning of the seasons. At the start of summer, for example, the Sun is farthest north in the sky for the entire year. And by the time the last rays of the Sun fade away, another marker is in good view: the Summer Triangle.

    The brightest point of the triangle is Vega, in Lyra, the harp. It’s high in the east-northeast as darkness falls. It’s one of the brightest stars in all the night sky, so you just can’t miss it. And it’s a close neighbor, at a distance of just 25 light-years — one of the reasons it looks so bright.

    Well to the lower left of Vega, look for the faintest point of the triangle, Deneb. It marks the tail of Cygnus, the swan. Its fainter appearance is deceiving, though. The star is a supergiant, so it’s tens of thousands of times brighter than the Sun. It looks fainter than Vega only because it’s about 1400 light-years farther — one of the most-distant stars that’s easily visible to the eye alone.

    Finally, look far to the lower right of Vega and Deneb for the triangle’s southernmost point, Altair, in Aquila, the eagle. It’s the closest member of the triangle, just 17 light-years from Earth. It’s actually the least impressive member of the triangle. Even so, it’s bigger and heavier than the Sun, and a good bit brighter, helping it stand out as a member of the season’s signature star pattern: the Summer Triangle.

    Tomorrow: noticing the tug of an unseen planet.


    Script by Damond Benningfield

    Sunday, June 25, 2017
    Triangulating the summer sky
  • Orange Triplets

    A system of three orange stars is in the south-southwest at nightfall, not far to the lower right of the bright planet Saturn. 36 Ophiuchi looks like a single, faint point of light. It consists of three stars that are smaller and cooler than the Sun.

  • Orange Triplets

    Stars come in all types. One of the most common types is the orange dwarf — which is also one of the hardest to see. Orange dwarfs are somewhat smaller and cooler than the Sun. And they’re not quite as bright, which is why so few are visible to the unaided eye. Tonight, though, you can see not one, not two, but three orange dwarfs all at once.

    This triple star is known as 36 Ophiuchi. It’s just 19 light-years from Earth. Its two brightest stars each emit about a quarter as much visible light as the Sun does. The two stars orbit each other every 500 years, on a highly elongated path. Yet another orange star stands a good distance away from these two, giving 36 Ophiuchi a grand total of three orange dwarf stars.

    Such stars account for about 10 percent of all the stars in the galaxy. They’re so faint, though, that none is prominent in the night sky. Fortunately, because 36 Ophiuchi is so close, it’s one of the few orange dwarfs you can see without binoculars or a telescope.

    You will, however, need a star chart and a good, dark, moonless night, like tonight, away from city lights. 36 Ophiuchi is quite low in the south-southeast at nightfall, and due south around midnight, not far to the lower right of the bright planet Saturn. And if you can’t get away from the city, binoculars will bring the system into view — a triple orange dwarf that’s among our nearest neighboring stars.


    Script by Ken Croswell, Copyright 2016

    Saturday, June 24, 2017
    Three orange stars for the price of one
  • New Moon

    The Moon is “new” today, as it crosses the imaginary line between Earth and Sun. It will return to view as a thin crescent on Sunday evening, quite low in the west shortly after sunset.

  • Storm Damage

    Solar storms can create all sorts of trouble. They can knock out power grids, cripple orbiting satellites, and disrupt airline schedules and radio communications. And a team of scientists is looking at one other possible form of trouble: stranding events for whales, dolphins, and porpoises.

    These marine mammals, known as cetaceans, have internal compasses to help them navigate the world’s oceans. But solar storms are caused by short-circuits in the Sun’s own magnetic field. They can send massive clouds of charged particles toward Earth. These particles cause problems when they hit Earth’s magnetic field.

    The researchers suggest that one of those problems is interfering with the navigation of cetaceans.

    These animals sometimes strand themselves on the beach, either individually or in groups. There are many possible causes for the strandings, including diseases and storms. And sonar causes some strandings by interfering with the animals’ internalsonar, which they use to locate both prey and others of their own kind.

    Because the animals have an internal compass to sense Earth’s magnetic field, though, the researchers suggest that solar storms could be another cause of strandings. The team will comb through many years of observations of strandings and solar storms. They’ll compare the two to see if there’s a relationship — perhaps identifying another threat to these beautiful creatures.

    Tomorrow: three stars for the price of one.


    Script by Damond Benningfield

    Friday, June 23, 2017
    Risks from storms on the Sun
  • Eltanin

    Eltanin, an Arabic name that means “the serpent,” is the brightest star of Draco, the dragon, which is high in the north on summer evenings. Eltanin is as bright as the nearby North Star, Polaris.

  • Quiet Sun

    The Sun has been unusually “quiet” in recent years — the quietest in more than a century. And forecasters say there’s a good chance it’ll be even quieter over the next few decades.

    The Sun’s level of activity is determined by its magnetic cycle. On average, a cycle lasts about 11 years. The Sun is most active at the cycle’s peak, when it can be covered by dozens of sunspots and produce big outbursts of energy and charged particles. These outbursts can knock out power grids on Earth, damage orbiting satellites, and disrupt our technology in many other ways.

    Astronomers have been watching the solar cycles for four centuries. And they’ve been counting them since 1755. Right now, we’re near the end of Cycle 24.

    The most active solar cycle ever recorded peaked in the early 1960s. The cycles since then have been weaker. And the current one is the weakest since the early 1900s. Over the last few years, in fact, there have been dozens of days when there were no sunspots at all.

    And scientists say that trend could continue. Early forecasts call for Cycle 25 to be even weaker than the current one, with Cycle 26 weaker still. In fact, we could be heading into a period of weakness that could rival the Maunder Minimum — a period in the 17th and 18th centuries where the Sun was almost comatose. If so, then the Sun could remain quiet through the end of the century — giving our technology a break from our star’s disruptive effects.


    Script by Damond Benningfield

    Thursday, June 22, 2017
    A long period of rest for the Sun
  • Hot Halo

    Clouds of charged particles form a halo around the Perseus galaxy cluster in this combined radio and optical view. The radio view, in red, is produced by particles that are accelerated by collisions between galaxies or by powerful jets from supermassive black holes. The halo spans about 1.3 million light-years. [Gendron-Marsolais et al.; NRAO/AUI/NSF; NASA; SDSS]

  • Odd Ophiuchus

    Ophiuchus, the serpent bearer, passes high across the south tonight, above Sagittarius and Scorpius. Ophiuchus is the only one of the 13 constellations along the Sun’s path that is not a member of the zodiac.

  • Busy Sun

    The Sun is one of the steadiest of all stars. Even so, its energy output does vary — by less than a tenth of one percent. This variation takes place over an 11-year cycle that’s tied to changes in the Sun’s magnetic field. But cycles aren’t the same. The most intense yet seen peaked in the early 1960s. And right now, we’re in the middle of the weakest cycle in more than a century.

    The Sun is a big, spinning ball. Because it’s made of gas, different layers and different latitudes rotate at different rates. That causes the Sun’s magnetic field to get twisted and tangled. Lines of magnetic force poke through the surface, creating dark sunspots, giant eruptions, and other activity. When the cycle peaks, the Sun can be covered with dozens of sunspots.

    Astronomers have been tracking sunspots for more than 400 years. In the late 1600s and early 1700s, they saw almost no sunspots at all — a period known as the Maunder Minimum. It corresponded to a period of unusually cold weather in North America and Europe. Four of the five busiest cycles they’ve seen have happened in the last 75 years or so.

    By looking at tree rings, and at ice cores from Antarctica and Greenland, scientists have estimated the intensity of solar cycles going back more than 11,000 years. And they’ve found that the peaks of the mid-20th century are the strongest in at least 8,000 years.

    Over the last few cycles, though, that intensity has faded. More about that tomorrow.


    Script by Damond Benningfield

    Wednesday, June 21, 2017
    Some busy years for the Sun
  • Summer Solstice

    Summer begins tonight in the northern hemisphere, when the Sun stands farthest north for the year — a moment known as the summer solstice. The season lasts until the September equinox, when the Sun crosses the equator from north to south.

  • Summer Solstice

    The modern calendar is one of the oldest and most basic astronomical instruments. It measures the relative motions of Earth and the Sun. A day, for example, is the average length of time it takes the Sun to return to the same place in the sky: 24 hours. And a year is the time it takes Earth to make one complete turn around the Sun.

    So perhaps it’s not surprising that the calendar uses astronomical definitions for the seasons: key points in the Sun’s annual migration across the sky.

    Summer, for example, begins tonight here in the northern hemisphere, when the Sun stands farthest north for the year — a moment known as the summer solstice. And the season lasts until the September equinox, when the Sun crosses the equator from north to south. The solstice also marks the longest day of the year.

    For centuries, the calendar was timed to match the seasons as well. In Great Britain and its American colonies, for example, the year began at the spring equinox in March. It wasn’t until 1752 that the British calendar was changed to move the start of the year to January 1st.

    Of course, astronomy isn’t the only way to mark the seasons. Meteorologists, for example, have a slightly different range for summer: from the first of June through the end of August. And families follow the school calendar, in which summer also spans June through August — different ways to mark the cycles of the Sun.

    We’ll have more about the Sun tomorrow.


    Script by Damond Benningfield

    Tuesday, June 20, 2017
    Marking time by the motions of the Sun
  • Moon and Venus

    Venus, the brilliant “morning star,” will stand to the left of the Moon at first light tomorrow, and about the same distance to the upper right of the Moon on Wednesday.

  • Moon and Venus

    One of the loveliest events in the night sky is a close alignment of the Moon and the planet Venus. And just such an alignment takes place the next couple of days before and during dawn. Venus, the brilliant “morning star,” stands to the left of the Moon at first light tomorrow, and about the same distance to the upper right of the Moon on Wednesday.

    A close alignment between two or more astronomical objects is called a conjunction. The Moon has several of them every month, because it makes a full loop against the background of stars and planets.

    And it’s always the same roster of companions, because the Moon follows a narrow path across the sky. In fact, it stays close to the Sun’s path, known as the ecliptic. Several bright stars lie near the ecliptic, and so do the bright planets. So the Moon swings past each of these objects every month, staging beautiful conjunctions.

    The Moon also has conjunctions with the Sun. Most months, these conjunctions are invisible. The Moon’s path is tilted just a bit with respect to the ecliptic, so the Moon usually swings just above or below the Sun, where it’s lost in the glare.

    In August, though, the Moon will be crossing the ecliptic just as it passes the Sun. That will create the most beautiful of all astronomical encounters: a total solar eclipse. Its path will cross the United States from Oregon to South Carolina — a beautiful conjunction between the Moon and Sun.

    More about the Sun tomorrow.


    Script by Damond Benningfield

    Monday, June 19, 2017
    Beautiful encounters with the Moon
  • A Civil Sky

    Evening twilight descends on McDonald Observatory and the Davis Mountains of West Texas. This depicts civil twilight, when the Sun is no more than six degrees below the horizon, and the sky is still fairly bright. Full astronomicaltwilight, when the Sun is between 12 and 18 degrees below the horizon, will come later, providing skies that are dark enough to allow astronomers to begin staring into the depths of the universe. [Damond Benningfield]

  • Approaching Summer

    Summer arrives on Tuesday night, with the summer solstice. Tuesday and Wednesday will be the longest days of the year here in the United States — the greatest intervals between sunrise and sunset.

  • Twilight

    Summer arrives on Tuesday night, with the summer solstice. Tuesday and Wednesday will be the longest days of the year here in the United States — the greatest intervals between sunrise and sunset. But the light of day doesn’t begin or end at those moments. Sunlight filters into the sky long before the Sun breaks the horizon in the morning, and long after it disappears in the evening.

    These periods are known as twilight. But there are several versions of twilight, all of which depend on how far below the horizon the Sun is.

    What most of us think of as twilight is known as civiltwilight. In the evening, it begins at sunset and ends when the Sun has dropped six degrees below the horizon — about the width of three fingers held at arm’s length. There’s still light in the sky at the end of civil twilight, but not enough for everyday activities, such as driving.

    When the Sun is between six and 12 degrees below the horizon, it’s nauticaltwilight. When sailors navigated by the stars, they could begin to see stars during this period, but they could also still see the horizon, so it was a good time to take readings.

    Finally, when the Sun is between 12 and 18 degrees below the horizon, it’s astronomicaltwilight. It’s dark enough for astronomers to observe brighter stars and planets, but not dark enough to see faint galaxies and nebulae.

    When astronomical twilight ends, the sky’s just about as dark as it’s going to get — the darkness of another night.


    Script by Damond Benningfield

    Sunday, June 18, 2017
    Dark, darker, and darkest skies
  • Sunrise and Sunset

    The timekeeper for the United States, the Naval Observatory, defines sunrise and sunset as the moments when the center of the Sun is physically 50 minutes of arc below the horizon, which is less than the width of your finger held at arm’s length.

  • Sunrise and Sunset

    In the modern world, there’s a rule, a regulation, or a definition for just about everything. And that includes such everyday events as sunrise and sunset.

    There are several ways to think of these daily bookends. They can be the points at which the Sun is bisected by the horizon — half in view, half not. Or they can be the points when the Sun has dropped below the horizon. Or, in the modern definition, they can be the points when the Sun drops from view.

    You might think those last two would be the same, but they’re not. Earth’s atmosphere acts as a lens, bending the Sun’s rays. So when you see the Sun standing just atop the horizon, it’s actually below the horizon, but the atmosphere has projected an image of the Sun into view.

    So the official timekeeper for the United States, the Naval Observatory, defines sunrise and sunset as the moments when the center of the Sun is physically 50 minutes of arc below the horizon — a bit less than one degree. That accounts for the size of the Sun itself, and the “bending” properties of the atmosphere.

    The atmosphere can bend the Sun’s rays at different angles at different times, though. So the predicted times of sunrise and sunset can be off by a minute or so. And when the Sun rises and sets at a low angle to the horizon, they can be off by several minutes.

    And even when the Sun does drop from sight, the atmosphere scatters its light back into view, producing twilight. More about that tomorrow.


    Script by Damond Benningfield

    Saturday, June 17, 2017
    Defining the day’s “bookends”
  • Big Changes

    Hydra, the water snake, slithers quite low across the southwest at nightfall. One of its treasures is the binary system V Hydra. Its main star is old and puffed up. It expels gas into space, which the second star grabs, then shoots out like cannonballs.

  • Big Changes

    For most of a star’s long life, things happen slowly. A star like the Sun, for example, steadily converts hydrogen to helium in its core for about 10 billion years. At the end of that run, the pace picks up. And in the star’s final, colorful act, major changes can happen over just a few centuries.

    One star that may be going through that last act is in Hydra, the water snake, which slithers quite low across the southwest at nightfall.

    V Hydra appears to consist of two stars. The main star is the same mass as the Sun. It’s at the very end of its life, though, so it’s puffed up to more than 400 times the Sun’s diameter. The star pulses in and out like a beating heart, and it’s ejecting a lot of gas into space.

    The second star orbits the bigger one every eight and a half years on a stretched-out path. When it gets close to the bigger star, it grabs some of the expelled gas, forming a disk. When enough gas builds up, some of it may be flung back into space as a “cannonball” — a blob that’s twice as heavy as the planet Mars.

    So today, astronomers are seeing big changes in V Hydra every few years. And the changes could come even faster in the not-too-distant future. The bigger star may soon expel all of its outer layers of gas into space, forming a colorful nebula. The companion may sculpt it into an exotic shape — perhaps an hourglass, or even a butterfly. It’s a process that could play out very soon — a fast-paced change for a Sun-like star.


    Script by Damond Benningfield

    Friday, June 16, 2017
    A Sun-like star picks up the pace
  • Blowing Bubbles

    A multi-wavelength image shows the Ring Nebula, the last gasp of a dying star. As the nuclear reactions in its core began to slow down, it started blowing strong winds into space, which form the dark red rings and arcs, shown in infrared light. Later, the star began expelling its outer layers in bulk, forming the brighter doughnut-shaped structure at center. Different colors show different chemical elements: red is hydrogen, for example, while blue is oxygen. The expanding nebula probably is shaped like a barrel, and we're looking through its open end. In a few thousand years, the nebula will dissipate, leaving only the star's hot but dead core, known as a white dwarf. [NASA/ESA/C.R. Robert O’Dell (Vanderbilt)/G.J. Ferland (Kentucky)/W.J. Henney and M. Peimbert (UNAM)]

  • Planetary Nebulae

    The last gasp of a dying star climbs the eastern sky on June evenings. The Ring Nebula is not far to the lower right of the brightt star Vega, which is about halfway up the sky as night falls. The nebula consists of the ejected outer layers of a once-normal star.

  • Planetary Nebulae

    When a Sun-like star dies, it leaves a brilliant corpse — a glowing shell of gas that can look like a ring, a box, or even an hourglass. The colorful object doesn’t last forever, though — it slowly fades from sight.

    Such a beautiful corpse is known as a planetary nebula — not because it has anything to do with planets, but because it can look like a planet when seen through a telescope. And one of the brightest and best known is in good view at this time of year.

    The Ring Nebula is not far to the lower right of the brilliant star Vega, which is about halfway up the eastern sky at nightfall.

    Long-exposure images show an oval that’s bright red and yellow on the outside, and pale blue and green in the center.

    The nebula began forming several thousand years ago, as its star stopped producing nuclear reactions in its core. The core began to shrink and get much hotter. A couple of thousand years ago as seen from Earth, radiation from the core began pushing away the star’s outer layers. As they expanded into space, energy from the superhot core caused them to glow — forming the Ring Nebula.

    The nebula is probably shaped like a barrel or a thick doughnut, and we’re looking straight through the hole in the middle. Other planetary nebulae may have companion stars that sculpt them into more exotic shapes, from hourglasses to butterflies — creating beautiful final acts among the stars.

    We’ll talk about the possible birthof a planetary nebula tomorrow.


    Script by Damond Benningfield


    Thursday, June 15, 2017
    A star’s colorful demise
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