What Is The Evidence

[Bazile]

How do Astronomers know that there are as many stars in the universe as there are grains of sand on the earth ?

Also how do they know that the milky way is a spiral galaxy ?

[LazyGun]

Telescope, and extrapolation :)

This recent post from JJ talks about this in some detail, if memory serves..

http://www.theanswerbank.co.uk/Science/Question1216169.html

[Bazile]

ah right - guesswork

[Nibble]

This explains far,far,far better than a simple AB'er than I can, but here's the link...

http://en.wikipedia.org/wiki/Galaxy

[LazyGun]

No, not guesswork. Reasoning. based upon observation and experiment - you know, the scientific method?

[FredPuli43]

Since when has making logical deductions from observable evidence been "guesswork" bazile ? If I let go of an apple and I deduce that it will fall towards the ground, is that guesswork?

[jake-the-peg]

Well estimation rather than guesswork - it's based on evidence.

By the way there's as many stars in a single galaxy as grains of sand in a beach.

OK it depends on the beach but if you know the size of a beach and the size of a grain of sand you can work out the number of grains right? Easy? (Well a bit more complex there are things like packing fractions etc.)

Well a galazy is somewhat similar, you can work out the average interstellar distance and the size of the galaxy and come up with an answer in a similar way - you can even check it by weighting the galaxy - you know how fast it rotates and how it moves from spectrascopic measurements so you can see how much it weighs, you have a fair idea of the average weight of a star so you can make sure the answer makes sense. You can also do the same thing with the luminosity of a galaxy and get a similar answer.

As for the shape of our galaxy - remeber we don't just observe the sky with light telescopes, infrared telescopes see through dust and give us a much better picture

[Bazile]

My answer was T-I-C

But seriously , how would i answer a kid asking me the question - is there a 'simple ' explanation of the deductions ?

[Bazile]

Right - i'll tell little johny to go ask his mum

[LazyGun]

heh - tell Johnny is reasoning, based upon telescopic observations over decades if not centuries, and calculated estimations ( reams and reams of paper rather than the back of a fag packet)

Rather more difficult might be trying to explain standard candles and cosmic distance though :)

Take a look at that video link from JJs post - its actually pretty good, I think..

[Teddio]

Consider a beach 5km long, 100m wide with a depth of sand of 1m
Volume of sand= 5000 X 100 X 1 = 500 000 cubic metres
= 500 000 000 000 000 cubic millimetres

There are 100 grains of sand in 1 cubic millimetre. Therefore as an estimate:

Number of grains on the beach = 50 000 000 000 000 000

Number of stars in an average galaxy = 100 000 000 000 (estimate)

When you look at a distant galaxy you can only see the brightest stars (such as Cepheid Variables) You can't see the most numerous stars, the Red Dwarves (so how do we know they are there?)

These are estimates with uncertainties of the order of +/- 1000%

There are approximately the same number of galaxies as there are stars in a galaxy, so the number of stars in the Universe is 10 to the power of 22 (again with an uncertainty of several hundred percent)



Would someone please check my figures ?

[vascop]

Bazile://ah right - guesswork//
No definitely not guesswork. It is an estimation based on observations.

[I_Hate_Infinity]

1) An appetite for an answer to a question

2) Constructing/Inventing apparatus to observe subject

3) Hours of observation, data collection and statistical analysis

4) Publication of conclusions to relevant property(s) of relevant subject(s) with free access to all data for public scrutiny and a reasonable hypothesis that conform to know physical properties of the Universe.

...Not all fun being a scientist, but like and explorer, they have the gratification of being the first person to see or theorise the answer to the question we wanted to know. Priceless

IHI