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Squished stars have researchers in a spin

Zoomed-in view of the star Regulus

A "zoomed in" view of the star Regulus. Measurements of the temperature of the star's poles and equator have shown up flaws in a century-old astronomical theory about hot, fast-spinning stars.

  • Hot stars spin so fast that they become slightly flattened
  • Theory predicts temperature differences between poles and equator
  • New measurements show the theory has major flaws

THE HOTTEST STARS IN THE UNIVERSE spin so quickly that they become a bit squished at their poles and dimmer around their middle.

But the 90-year-old theory that predicts the extent of this “gravity darkening” phenomenon has major flaws, according to a new study led by University of Michigan (U-M) astronomers.

The von Zeipel law, named for its creator Swedish astronomer Edvard Hugo von Zeipel, has been used for the better part of a century to predict the difference in surface gravity, brightness and temperature between a rapidly rotating star’s poles and its equator.

Using a technique called interferometry, the U-M researchers essentially ‘zoomed in’ to take close-up pictures and measurements of the giant star Regulus.

If Regulus were spinning just a few percent faster, it would fly apart.

The astronomers found that the actual difference in temperature between its equator and poles is much less than the old theory predicts.

Measurements don’t match the theory

“It is surprising to me that von Zeipel’s law has been adopted in astronomy for such a long time with so little solid observational evidence,” said Xiao Che, a doctoral student in the Department of Astronomy who is first author of a paper on the findings to be published in Astrophysical Journal on April 20.

It’s important to get the numbers right, says John Monnier, an associate professor in the U-M Department of Astronomy.

“In some cases, we found a 5,000-degree Fahrenheit [2,750 degrees Celsius] difference between what the theory predicts and what our actual measurements show,” Monnier said.

CHARA telescope array

Combining the light from the six CHARA telescopes (arrowed) with the Michigan Infra-Red Combiner, gives astronomers a virtual telescope 100 times as big as the Hubble Space Telescope.

“That has a big effect on total luminosity. If we don’t take this into account, we get the star’s mass and age and total energy output wrong.”

Zooming in with virtual telescope

Monnier led the creation of the Michigan Infra-Red Combiner (MIRC) instrument that was used to take the measurements. MIRC combines the light from four telescopes at the CHARA array at Georgia State University, producing a virtual telescope 100 times larger than the Hubble Space Telescope.

This interferometry technique enables astronomers see the shape and surface characteristics of stars. Without this technique, stars look like mere points of light even through the largest telescopes.

In this case, zooming in on Regulus let the researchers measure its poles and equator temperatures separately.

“Normally, you would just be able to get an average temperature,” Monnier said.

So where did von Zeipel go wrong? Monnier believes his Swedish predecessor didn’t take into account circulation patterns on stars that are not unlike wind patterns on Earth.

“The Earth has a hot equator and cold poles and that causes air circulation,” Monnier said.

“The hot air wants to flow toward the poles and equilibrate, bringing the temperatures closer together. This is a source of some weather patterns on Earth.”

Adapted from information issued by the University of Michigan. Regular image courtesy Xiao Che.

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What’s up? Night sky for April 2011

Amateur astronomers using a telescope at dusk

Autumn nights are ideal for stargazer. Autumn mornings too, with some planets making a welcome return to our pre-dawn skies.

LIKE LAST MONTH, Venus is the star of the show in April, high in the eastern sky before sunrise. You can’t miss it—it is big and bold and brilliant.

Mercury makes a reappearance in the eastern morning sky from mid-month. As the April progresses, you’ll notice it rising higher and higher in the sky.

Ditto for Mars, which has been lost from view in the glare of the Sun for a while, but is making a reappearance in the morning sky this month, very low down in the east.

The largest planet in the Solar System, Jupiter, will also be low down on the eastern horizon before sunrise during April.

So that’s four planets all quite near to one another in roughly the same part of the sky! Definitely worth getting up early for.

Shifting to the evening sky, and the ringed planet Saturn will be bright and bold in the eastern sky after sunset.

Except where indicated, all of the phenomena described here can be seen with just the unaided eye. And unless otherwise specified, dates and times shown here are for the Australian Eastern Standard Time zone, and sky directions are from the point of view of an observer in the Southern Hemisphere.

April 1

Have a look out to the east before sunrise and you’ll see the Moon just below the planet Venus. You can’t miss Venus—apart from the Moon and the Sun, it’s the brightest thing in the sky!

April 2

Today the Moon will be at the farthest point in its orbit, called apogee, at a distance from Earth of 406,657 kilometres.

Also, you’ll notice that compared to yesterday the Moon has moved downwards in the sky and away from Venus. Now, it will be closer to the planet Mars, which is fainter than Venus and a ruddy colour.

April 3

New Moon occurs today at 11:32pm Sydney time (April 3, 14:32 Universal Time).

April 4

The planet Saturn reaches opposition today. This is a fancy way of saying that, from an Earth-bound observer’s point of view, it is in the opposite side of the sky to the Sun. In other words, if you could look down on the Solar System from above, the Sun, Earth and Saturn would be in a line, with Earth in the middle. Opposition generally is a good time to view a planet.

Stargazers with telescopes

Take the opportunity to get out and do some stargazing before the weather becomes too cold

April 7

The planet Jupiter reaches conjunction with the Sun today, which is a fancy way of saying that it is on the other side of the Sun as seen from Earth…although, being on the other side of the Sun, we can’t actually see it. You can think of conjunction as being the opposite of opposition (see April 4 above), so to speak. If you could look down on the Solar System from above, you’d see Jupiter, the Sun and the Earth in a line, with the Sun in the middle.

April 10

Today, the innermost planet, Mercury, reaches ‘inferior conjunction’. This is another kind of conjunction, again signifying that certain celestial bodies are in a line. In this case it is the Sun, Mercury and the Earth in a line, with Mercury in the middle. This makes it impossible to see the tiny planet, as it is lost in the glare of the Sun.

On certain special occasions, the tilt of Mercury’s orbit means that it can be seen crossing the face of the Sun when it is at inferior conjunction, in an event known as a transit. That’s not the case here—the next one will be in 2016.

April 11

It is First Quarter Moon today at 9:05pm Sydney time (April 11, 12:05 Universal Time). First Quarter is a good time to look at the Moon through a telescope, as the sunlight angle means the craters and mountains are throwing nice shadows, making it easier to get that 3D effect.

April 14

The almost-full Moon will appear close to the star Regulus in this evening’s sky. Regulus is a large, blue star, and is the brightest star in the constellation Leo.

April 17

Today the Moon will be at the closest point in its orbit, called perigee, which is the opposite of apogee (see April 2). The distance between the two bodies today will be 358,090 kilometres.

Still on the Moon…take a look slightly below and to the left, and you’ll see a bright whitish-yellowish star. Well, that’s not a star, it’s the planet Saturn. If you have a telescope, or have a friend who has one, zero in on it and you’ll get a lovely view of its magnificent rings.

April 18

Full Moon occurs today at 11:44am Sydney time (April 18, 02:44 Universal Time). You won’t actually be able to see the Moon at that time, as it will be below the horizon. But take a look at sunset, and you’ll see that as the Sun sets in the west, the big, bright Moon will rise in the east.

April 20

Two planets get to say hello to each other this morning. In the eastern sky before sunrise, Mars and Mercury will appear close to each other. Be warned, though, that they will be very low down toward the horizon, so you’ll need a clear view if you’re to see them.

Over the next few weeks, four planets will be seen together in the morning sky. Here’s a short video from Tanya Hill, astronomer at the Melbourne Planetarium, explaining what we can expect to see:

April 21

Today will be an interesting to demonstrate to yourself how the Moon moves through the sky. If you get up in the morning while it’s still dark, take a look slightly above the Moon and you’ll see a ruddy-coloured star. That’s Antares, the brightest star in the constellation Scorpius. Remember that view. Now, go back outside later today in the mid-evening and find the Moon again, and you’ll see that the separation between the two has increased. This shows you how the Moon has trundled along a bit in its orbit throughout the day.

April 25

It is Last Quarter Moon today at 11:47pm Sydney time (14:47 Universal Time).

April 30

Today the Moon will again reach the farthest point in its orbit, apogee, at a distance from Earth of 406,038 kilometres. Take a look out to the east this morning and you’ll see the crescent Moon with Venus, Mercury, Mars and Jupiter (refer to the video above).

If you have any questions or comments on the night sky, we’d be happy to answer them. Please use the Feedback Form below. Happy stargazing!

Images courtesy IAU.

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Planetary line-up this week

Diagram of a planetary line-up May 19-22.

Diagram of the planetary line-up May 19-22, as seen from the perspective of an observer in the United States.

  • Moon, two planets & star line up
  • Happens from May 19-22
  • Easy to see – don’t need a telescope

The bright star Regulus joins the Moon and the planet Mars to form a beautiful lineup high in the southern sky (for US observers; in the northern sky for S. Hemisphere observers) at nightfall May 19-22, according to the editors of the University of Texas’ StarDate magazine.

And the best part of this stargazing spectacle is that you won’t need a telescope or binoculars to enjoy it. All you need is your eyes and a clear sky!

Starting on May 19, Mars will be in good view above the Moon as night falls. Regulus shines to the left of Mars, slightly higher. Regulus is the brightest star of Leo, the lion, and is more massive, hotter, and brighter than the Sun. Mars looks like a bright orange star.

On May 20, Regulus will be a little to the upper right of the first-quarter Moon, with bright orange Mars farther to the Moon’s right. The trio will form a wide, skinny triangle. They will be high in the sky at nightfall, and drop from view by around 2:00am

On May 21, the Moon will lie between Saturn (due south) and Mars (in the southwest) at nightfall. Saturn looks like a bright golden star.

By May 22, the Moon shines below Saturn, high in the south at sunset.

Adapted from information issued by StarDate magazine.

Moon, two planets & star line up

Happens from May 19-22

Easy to see – don’t need a telescope

The bright star Regulus joins the Moon and the planet Mars to form a beautiful lineup high in the southern sky (for US observers; in the northern sky for S. Hemisphere observers) at nightfall May 19-22, according to the editors of the University of Texas’ StarDate magazine.

And the best part of this stargazing spectacle is that you won’t need a telescope or binoculars to enjoy it. All you need is your eyes and a clear sky!

Starting on May 19, Mars will be in good view above the Moon as night falls. Regulus shines to the left of Mars, slightly higher. Regulus is the brightest star of Leo, the lion, and is more massive, hotter, and brighter than the Sun. Mars looks like a bright orange star.

On May 20, Regulus will be a little to the upper right of the first-quarter Moon, with bright orange Mars farther to the Moon’s right. The trio will form a wide, skinny triangle. They will be high in the sky at nightfall, and drop from view by around 2:00am

On May 21, the Moon will lie between Saturn (due south) and Mars (in the southwest) at nightfall. Saturn looks like a bright golden star.

By May 22, the Moon shines below Saturn, high in the south at sunset.

Adapted from information issued by StarDate magazine.