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back to article Stars say relativity still works

The Special Theory of Relativity may be under re-evaluation following CERN’s astonishing neutrino observations, but over in the world of astronomy, general relativity has had another reconfirmation from the Neils Bohr Institute at the University of Copenhagen. Radek Wojtak, Steen Hansen and Jens Hjorth have published in Nature …

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You know, my best guess is that so much is known about galaxies, you can probably tell the mass to within a couple percent just by observing one. I wonder if they included dark matter in their calculations. Probably did.

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TheElder

Some editing is required. The galaxies in question are a good deal farther than 8000 light years from us. That's just a stroll in the park. The 8000 number in the original article refers to the number of galaxy *clusters* that were tabulated and averaged to detect the influence of gravity on the light emitted by the clusters themselves.

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"El Reg would like to know how that’s accomplished, commenters"

Count the galaxies in the cluster. Based on type of each we have a fair idea of how many stars any particular type contains. M31, for instance, has about 1 trillion stars (10^12) as determined by the Spitzer telescope. Since we have a very good idea how distant it is and how bright it is we can extrapolate what other similar looking galaxies will mass even when much further away. The same applies for other types (shapes and sizes) of galaxies that we can observe relatively close by for calibration.

With this sort of information we can estimate the total mass fairly closely, maybe give or take 50%. Of course, there is still the question of "dark matter" and "dark energy".

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But...

What's the weight in Elephants/Bulgarian funbags/*insert appropriate measure*?

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Easily measurable

To the kitchen scales!

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Happy

Er, -

the weight is zero, in all units...

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Headmaster

>we can estimate the total mass fairly closely, maybe give or take 50%

If your accounts department only paid your salary 'give or take 50%' would you think that was 'fairly close'?

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Gravity makes weight, mass makes gravity

No, as there is mass within the clusters, there is obviously gravity. The gravity from any one massive body accelerates all other massive bodies, leading to weight. Consequently everything with mass has weight, the weight being affected by the gravitational frame of reference.

So whatever the answer is, it is not zero. ;-)

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hmm

Depends on if they were giving or taking :)

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Boffin

No, but in some areas of physics, 50% can be a great margin of error, some things have margins of error which are orders of magnitude.

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Astronomical

If my payment were astronomical, yes I would.

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if it was give, id be happy enough

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Takes me back to physics O level, part of the practical exam was estimating quantities - with a margin of error of 100%

estimate everything to be 0 for a pass.

:-)

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In astro, 50% is damn good.

For most things, you're happy just to get in the right order of magnitude. One of the reasons they work so hard to add more digits on the end of an AU is so they can trim the error margins when you have to multiply it by the billion billions that is your baseline to the next star.

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Boffin

"No, but in some areas of physics, 50% can be a great margin of error, some things have margins of error which are orders of magnitude."

Thing is, the difference between the measured speed of those neutrinos and the speed of light is a tiny percentage, so astronomical measurements are never going to come close to proving this one way or the other.

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"measured speed of those neutrinos"

This has nothing to do with speed of light or neutrinos - it's another test of General Relativity

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Irrelevant, 'give or take 50%' is good enough for things we know a lot more about like meteorology and investment economics so such a number on theoretical physics is indeed pretty close.

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Coat

How they did it...

First, they observed the amount of red-shift. They then used this to calculate the mass. They then used this calculated mass to predict the degree of red-shift that should occur. They then observed the amount of red-shift and... oh.. wait... um ......

(in related ponderings, use of trigonometry and solar parallax to calculate the distance of observable objects falls down at some quite close distance to the Earth - I haven't yet found a satisfactory explanation of how we can then know the distance of something that is further than that threshold distance... anyone ?)

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You can tell distant stars by how dim they appear. But don't different size stars put out different amounts of light? Yes, but they also put out different wavelengths of light too. Biggers stars put out more light, but they are a lot hotter and throw off shorter wavelengths. Don't worry about the red giants. Anywy, you can tell how much light they should be putting out by looking at the wavelength. Then by compairing that to the amount of light you actually see, you can tell the distance. At least, that's how I think its done.

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Boffin

I am not an astronomer, but if I remember correctly, the "standard candle" method is used. This compares the luminosity of known stellar objects at various distances. http://en.wikipedia.org/wiki/Standard_candle

Speaking of which, what are the El Reg official units for stellar luminosity?

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Anonymous Coward

Measure distance

It's quite simple to work out the distances really.

1. The universe is expanding in all directions.

2. Redshift correlates to speed differential (it's the doppler effect)

Therefore, objects further away will be going away faster and will thus have more redshift.

Calibrate by measuring a few nearby stars (which can be triangulated) and you're good to go.

Redshift is calculated by knowing that specific elements in a star will absorb light at very specific wavelengths, which then tells you both the chemical makeup and the size.

All quite clever really!

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Boffin

Not quite (AC 06:47)

You have to establish the value of the Hubble constant, which you can't do by measuring stars in the Milky Way using parallax (which can only be done out to ~100ly anyway) - their motion is determined by individual factors and the rotation of the galaxy - or even the closest (few million ly) galaxies - Andromeda is blue-shifted, moving towards us ready for collision in a few billion years.

For nearby galaxies (~100 million ly) we can use Cepheid variables, which are very bright variable stars whose absolute luminosity is directly related to their period - the astrophysics of this is well understood. For more distant galaxies, we can use a specific type of Supernova (1a), which is the result of a white dwarf accumulating hydrogen from a neighbouring star until a critical mass is achieved and they should therefore all have the same absolute luminosity. We can identify type 1a by the decay spectrum (the current Whirlpool galaxy supernova is of this type), but supernova physics isn't as well understood, so opinions of how bright these events actually are may vary.

The Hubble constant determines the age of the universe (1/H with some fudging for the way H is now believed to have changed over time), and the current value of close to 70 kilometers per second/Megaparsec fits the results of the cosmic microwave background quite well (phew!)

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FAIL

Standard units of stars

I think probably something like:

Brown dwarf; very dim. Units like 1 ashton kutcher or 3 Paris Hitons.

Um that's as far as I can take this. I'm a nerd and like Sheldon Cooper, I fall down on knowedge of film stars and popular culture.

The fail is for me.

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Anonymous Coward

what are the El Reg official units for stellar luminosity?

Measured in Milkman-eye-glints assuming a standard British 30 year old housewife in a baby-doll nightie circa 1972.

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Happy

If I remember correctly . . .

the parallax method gives you a 'local' population of satrs with directly measured sitances. This allow you to calculate absolute magnitudes (how bright the *really* are) for a range of star types. Within this group are a class of stars known as Cepheid variables whose brightness changes with a period dependent on their size. These standard candles then allow you to calculate distances for more distant stars and 'local' galaxies. From the population of galaxies you get a scale of mass/brightness which can then be used to calculate equivalent values for more distant galaxies where you can't pick out the standard candles.

If I remember correctly . . . .

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Boffin

There is a similar problem in particle physics, from what I remember from Uni 11 yrs ago, whereby detectors measure energy and thus the velocity and mass have to be intelligently guessed.

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Brown Dwarf = 1 Kutcher

Really bright, hot star (guess my knowlage is invert of yours - fail for me) = 1 Vorderman?

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One slight problem with that . .

is that you have some circular logic there. The redshift gives you the distance if you know the Hubble constant. But you need to know the distance of some (non-triangulated galaxies to calculate the Hubble constant.

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Happy

Presumably going all the way up the scale to supernova == 1 Brangelina

Bazinga!

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The parallax calculations depend on the baseline,

so it depends on how you take your baseline. The use of photographic plates and keeping precise records on time, location, and orientation of the telescope has allowed us to extend the baseline a fair piece. Not only can we get the AU as a baseline, but because we can approximate the sun's orbit around the center of the galaxy, we can extend the baseline beyond an AU.

That does still limit to relatively close stars, but from there you can start to work with relative/absolute magnitude to determine the distance to the star. The initial work there was done with binary pairs where you have an independent means of determining mass. But once that is established you work more with red-shifting. It all keeps feeding back on itself, but you get further and further away and become more and more comfortable that you assumptions and calculations are correct.

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Trollface

Speaking of which, what are the El Reg official units for stellar luminosity?

I would suggest the "Nuke", but this unit would be a bit too large : The sun would be a bit less than one milliNuke according to some american and japanese observers in 1945.

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"I haven't yet found a satisfactory explanation of how we can then know the distance of something that is further than that threshold distance... anyone ?)"

The overall redshift is directly correlated to the distance. What was measured in the above article is differences in the redshift from center to edges. Those differences are very much smaller than the average redshift of the cluster. It is the average redshift of each of the galaxies being similar that identifies the members of the cluster.

I must get back to my astrophotography. It's the first good night for same in quite a long while.

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Coat

Not done any astraphotography, but I did once get a good picture of a Vectra...

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Devil

A good picture of a Vectra?

Are you sure...?!

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You can estimate the mass of a cluster by looking at the velocities of the galaxies and the gas clouds in that cluster. These velocities also result in a shift of all spectral lines, to the blue when they move towards us and to the red when they move from us.

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Except that all spectra are red-shifted.

This is one of the observations which leads to the hyper-exapnding universe hypothesis. So you first have to get the average red shift, and then look at the relative red shifts to find what is blue-shifted compared to the average.

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Coat

IIRC, didn't the brightness of Cepheid variables provide some measure of the distances of galaxies containing them? Personally I find sky photographs rather confusing: individual stars, galaxies, clusters of galaxies all just projected onto a 2-d surface. Who is to say that a particular object in the photo isn't just a smudge on the telescope lens/mirror?

Need a good warm coat for astrophotography!

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No mention of

No mention of red shift due to absorbtion/retransmission through more galactic dust. Was that factored in?

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Reply to Full Mental Jacket

Reddening due to absorption by galactic dust is not the same as redshift. Reddening is a filtering effect - blue light is more likely to scatter, red light less so. More of the original red light reaches us than of the original blue light. In this case, information is lost because light of specific wavelengths never reaches us.

But redshift changes the colour of the light, by lengthening its wavelength. The original information is not lost, merely shifted down the spectrum.

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Probably not

Career astrophysicists are well known to be an ignorant, lazy, workshy bunch who only went into academia because the real world seemed too stressful. They probably don't know a whole lot about astrophysics, and they probably don't know anyone who does and even if they did they wouldn't ask them.

They probably just got drunk, scrawled down a bunch of random numbers in vomit and posted it off to the nearest journal, without even asking the omniscient polymath denizens of the commentardia to do any proofreading.

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I really hope CERN's observations are correct.

Largely for the spectacle of watching Prof Al-Khalili eat his own boxers.

http://twitter.com/#!/jimalkhalili/status/117160630527594496

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Go

Boxer shorts

He may organise a pair made of rice paper, worn outside an ordinary set of y-fronts.

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Calculating cluster weight

The weight of the individual Galaxies in the cluster is known as it's printed on the side of each bar.

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...

Niels Bohr.

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Windows

Only way to sort this argument..

..is to shoot yourself. Then, ask God directly. Sorry if I can't bring an answer sharpish...

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Anonymous Coward

can i suggest then....

..that you try to encourage as many religious types as possible to try and find the answer for us..

:-)

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The Virial Theorem

Initial estimates of galactic mass are based on the Virial Theorem.

Assuming that the only force in play is gravity and assuming that stellar velocities have more or less evened out ('virialised') from the galaxy's initial formation, then the Virial Theorem shows that the total kinetic energy is equal to half the potential energy. Total kinetic energy is proportional to total mass. Total potential energy (in a given configuration) varies as square of mass. So if the relative motion of the stars about the galaxy centre of mass is known - and information about their velocities is available from spectroscopic observations - by making reasonable assumptions about their distribution within the galaxy it is possible to estimate the total mass.

Unfortunately, observations of the variation of stellar velocities with distance from the centre are not in accord with this simple model. The prediction is that outside the main concentration of stars, velocities should fall off with the inverse square of distance. In fact, velocities deduced from spectographic observations appear pretty much constant to a considerable distance from the centre. Equally puzzling is the observations that stellar (linear) velocities are in the range of 150 to 350 km/S irrespective of large differences in the size of the galaxy they inhabit.

It is the discrepancy between the 'Keplerian model' (stars in a galaxy behaving in a similar way to planets around a star under the influence of inverse square law gravity only) and the observation of more or less constant velocity which gave rise to the idea of dark matter. This dark matter is assumed, by some miracle or other, to sit in the shape of a halo which is in precisely ihe right place to cause the observed pattern of stellar velocities.

A current rationalisation of the dilemma which some cosmologists favour comes from MOND - Modified Newtonian Dynamics. Here it is assumed that the inverse square law does not hold at very large distances.

For reasons which largely escape me, the alternative explanation proposed half a century ago by Hannes Alfvèn that electromagnetic processes are in play is generally treated as heresy.

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Alfven was an electrical engineer and (part-time) plasma physicist....

He proposed electromagnetic processes as the solution to virtually every problem he looked at.....

This partly explains why the 'physics establishment' were a little sniffy with him.

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Viral Theorem - Crekshun

"... outside the main concentration of stars, velocities should fall off with the inverse square of distance ..."

replace with

... velocities should fall off with the inverse square root of distance ...

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Windows

Abraham Maslow...

"When the only tool you have is a hammer, every problem begins to resemble a nail"

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