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back to article Streaking star leaves sweeping tail in its wake

A space-based observatory has captured images of a 13 light-year long tail stretching out behind a well known red-giant star, Mira. The pictures have come as a complete shock to astronomers: Mira has been gazed upon from Earth for almost half a millennium, but nothing like this tail has ever been seen before. A new ultraviolet …

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Baa!

"Might as well compare it to the speed of the average sheep (roughly 150,000 times as fast, we reckon)."

In Space no one can hear you Baa!

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Silver badge

Not so fast!

Actually Lucy, there *is* a speed of sound in space (and it's very high compared to Mach 1 in the Earth's atmosphere). This is because space is not (quite) empty, but travelling 'supersonically' has similar effects - matter can't get out of the way of the supersonic object and therefore gets compressed to (relatively) high temperature and pressure.

http://helios.gsfc.nasa.gov/qa_study.html

has more info (see #28).

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Rob

"Supersonic" is correct....

.... since the speed of sound in an ideal vacuum is 0 ms-1, anything travelling from our relative point of view in that medium is supersonic.

However, I take the point that the NASA press release was talking out of its arse.

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not so "Tch tch"

The Inter-Stellar Medium does contain matter, and the presence of a bow shock around the front of the star shows that it is travelling at a supersonic speed.

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Sheep miles per hour

Maybe an interesting new unit of mearurement.

My current favourites are the scone (as a measure of volume, as in "this rucksack can hold maybe 400 scones!") and the Furlong per Fortnight (unit of speed - light travels at approximately 1.8 terafurlongs per fortnight)

Any others?

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Steaking Star?

My first thought was "Not another Paris bloody Hilton story" (although the "sweeping tail" bit confused me).

The picture, though, is absolutely beautiful. Which just goes to prove that it's nothing to do with Paris Hilton. :)

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New SI Units

@Foz Hughes what about:

Parsec/grammes for torque?

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Anonymous Coward

The Speed of Sheep

"... the speed of the average sheep (roughly 150,000 times as fast, we reckon..."

Oh, how disappointing to see such a mistake in The Register!!!

FIRSTLY, just because the Yanks choose to muddy science with archaic measurements isn't reason for you to perpetuate that usage.

However, that's not a "mistake", as such - what I'm worried about is your speed of a sheep, which you have calculated wrongly.

A sheep, being chased by a wolf, at the equator, just after midnight, on the right day of the Earth Year, will be travelling many orders of magnitude faster than your reckoning.

By *my* reckoning, this sheep will be travelling at approximately 230,000 m/s, compared with this tired old sun which is only doing 133,000 m/s.

So, our humble woolly friend is *much* faster than this old red giant.

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Sheepspeed

I imagine the speed of a sheep in space would be very little since it would have died due to a lack of oxygen...

Unless of course the sheep had a space helmet (and some grass) - then presumably it would be able to propel itself using flatulence. Any offerings as to the speed of ass-gas?

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Simplifying NASA

Maybe NASA should stick to simpler statements next time?

Like “Ohhhh look, a weird glowing thing” or the old MIB favourite “ it’s light from Venus reflecting off a weather balloon”

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Bronze badge

Baah! Bumhug!

Determining sheep speed is a tricky thing depending on a number of variables; a grazing sheep barely manages 1 mile an hour, whereas sheep running together on one man and his dog will typically hit about ten miles an hour. Sheep in full flight from a wolf or a green wellied townie's dog can make twenty miles an hour plus but the fastest sheep are Welsh and to be seen on Friday nights avoiding the amorous advances of drunken, viagra snorting locals, to escape them, the sheep have to achieve twentyfive miles an hour plus as the locals hunt them on bicycles.

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Sheepspeed reference point

Dear anonymous (13:49GMT)

Left aside that the described circumstances IMO are not very typical for a sheep and thus you barely can call that the average sheep motion:

I assume you have chosen a point of reference that is not very usual for ground based animals on earth. Even if you were refusing to take the surface it runs on as reference i think you would rather take yourself as reference point than an astronomical reference like the sun (i assume you used that without doing the maths, excuse my ignorance) .

Taking yourself as reference point, you cannot really count earth rotation, only distance change. Thus your sheep is at the very most as fast as a fleeing sheep near you.

(I felt the need to defend my image of sheep as comfortably slow - even if i'm wrong, 133000m/s simply sounds not right for sheepspeed)

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Anonymous Coward

Sheepspeed reference point

232,000m/s, please!

133,000m/s was the dodgy-sounding speed given by these NASA morons.

My sheepspeed reference point is the galactic centre - sensible if we are comparing it to another astronomical body.

Maybe you need to ask exactly what reference point these idiots (who use miles-per-hour instead of scientific measurement) used to come up with this dubious "speed" for this red dwarf sun.

Is it the speed of the sun from its own tail? From our sun? From the galactic centre?

It's just another meaningless pseudo-scientific mish-mash unless they can provide the reference point and use proper scientific units of measurement.

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Anonymous Coward

yellow dwarf

in astronomy a dwarf is a type of star our sun is not a yellow

dwarf in fact it's not yet a dwarf of any kind shut up and go away.

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sheep in space

A sheep doesn't need to be alive to be travelling through space. Just as demonstrated in Monty Python's Holy Grail, dead farm animals can travel just as fast as those with heartbeats

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Silver badge

The Sun is a dwarf, Mira never was

Actually, the Sun IS a "dwarf" - since astronomers use the term to refer to a star that is on the main sequence; that is, one that has contracted to stable brightness from the protostar stage and is still fusing hydrogen prior to expansion into the "red giant" stage. Abnormally bright and large main-sequence stars, such as Rigel or Deneb, are referred to as giants or supergiants, but are still differentiated from "red giants / red supergiants", which are stars that have left the main sequence and begun fusing helium, neon, and carbon. The Sun is a normally bright star about half-way through its main-sequence life cycle, so is still a dwarf star. Add to that the fact that it's very small compared to a hot blue giant like Rigel or Deneb, and you can see that the Sun is a dwarf in the literal sense as well as the astronomical one.

Also, Mira was never a star like our own sun - it's way more massive than the Sun is or ever could be. In its main-sequence cycle, Mira would have been a hot blue giant like the above-mentioned stars; these turn into red supergiants like Betelgeuse, Antares, or Mira itself. Such stars are so immense that if they were placed were the Sun is, their surfaces would be out by the orbit of Uranus, and Earth would be just outside its inner core. As it is, stars like the Sun expand to become mere red giants like Arcturus or Aldebaran, whose size might extend to the orbit of the Earth or even Mars, but no larger than that.

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<Humour> It's not a Star!

It's the very first galactic comet! Like an ordinary comet, except instead of orbiting the sun, it's going around the galactic core. You can see the coma, the tail and everything. </Humour>

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Sheeposonic

So if a spaceship is traveling at 10 lightyears per second, and the sheep in the spacesheep is traveling at 10 kilometers per hour, how fast is the sheep traveling?

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space velocities, dwarf vs. giant stars, and GALEX Mira visualization

Re "Sheepspeed reference point" -- there are several common reference frames for measuring space velocities of nearby stars:

(a) relative to our Sun's motion

(b) relative to the "Local Standard of Rest" (google that), the motion of a circular orbit in the Milky Way plane that has the same radius as our Sun's orbit

(a) comes directly from measured quantities: the star's radial velocity, its apparent motion across the sky, and distance. But when I do that I get about 160 km/sec for Mira, so the NASA article's 130 km/sec might be relative to the LSR. Reporting a velocity relative to the Galactic center is certainly possible, but inconvenient -- the star is close to us and far from it -- so an astronomer wouldn't likely do it that way.

130 (or 160) km/sec is pretty fast as nearby stars go. 30-40 km/sec is more typical. (1 km/sec is just about 1 parsec/million years.) So Mira is on an unusual orbit -- most stars stay pretty close to the Milky Way plane, but Mira's orbit must carry it far above and below the plane. It's heading for galactic south at over 100 km/sec.

Re dwarf vs. giant stars, note that a "dwarf" star is just one that's still fusing hydrogen in its core, regardless of whether it's luminous or not, while a "giant" has run out of hydrogen there and is approaching the end of its life (and is generally brighter than it was as a dwarf).

Mira used to be a dwarf star like our sun -- and it's not all that different from how our sun will look in a few billion years. It's a *little* more massive (about 1.2 solar masses, says a recently-updated Wikipedia page). (It has a companion star in orbit around it, so a credible mass estimate is possible.)

At the ends of their lives, when they've run out of hydrogen in their cores, sun-sized stars swell up and go through several giant phases -- Mira is now an "asymptotic giant branch" (AGB) star, a late-phase giant. It's now very swollen, something larger than Mars' orbit, and losing its atmosphere at a great rate.

Rigel and Deneb are also giants -- their core hydrogen has been used up -- but are much more massive than our Sun, so (a) they're 'way more luminous (and were 'way more luminous back when they were dwarf stars too) and (b) their giant phases will end differently -- they'll become supernovae, while stars like Mira and the Sun won't.

Finally, dig this: For the GALEX web site (the satellite that made the observation of Mira's wake), Robert Hurt of Caltech made this lovely artist's-concept animation of how a wake like this might form:

http://www.galex.caltech.edu/MEDIA/2007-04/video.html

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