Thread: Space: STARS

I have just started the space unit in school and have 3 questions. I hope someone can answer and explain. I would appreciate!

1) "If you look into the night sky, you will see a cloudlike band of stars that stretches across the sky. Because of its milky appearance, this part of the sky is called the "Milky Way". The "Milky Way" is the disk of the "Milky Way Galaxy".

I don't get this part! Are "Milky Way" and "Milky Way Galaxy" the same thing or are they different? What is a "disk" and what does it mean by "The 'Milky Way' is the disk of the 'Milky Way Galaxy'."?

For example, in the following picture, which part is the disk?


2) As you observe the night sky, why do stars apear to move westward across the sky?

I would say earth's rotation causes stars to appear moving westward around a central point (Polaris). And earth's revolution causes them to shift slightly to the west each night, over a period of a year. Am I right?

3) "The earth is 4.6 billion years old. How many revolutions around the center of galaxy has it travelled? The sun is located in one of the spiral arms and completes one revolution in about 200 million years."

For the sun, I can calculate that it has completed 460000000/200000000=23 revolutions around the center of the galaxy? When the sun completes one revolution around the center of the galaxy, does the earth completes one revolution around this center, too?

Originally Posted by kingwinner

I have just started the space unit in school and have 3 questions. I hope someone can answer and explain. I would appreciate!

1) "If you look into the night sky, you will see a cloudlike band of stars that stretches across the sky. Because of its milky appearance, this part of the sky is called the "Milky Way". The "Milky Way" is the disk of the "Milky Way Galaxy".

I don't get this part! Are "Milky Way" and "Milky Way Galaxy" the same thing or are they different? What is a "disk" and what does it mean by "The 'Milky Way' is the disk of the 'Milky Way Galaxy'."?

For example, in the following picture, which part is the disk?

Houm...

"Milky Way" was the name that the ancient Greek gave to a cloud of stars crossing the night sky. It has been later when we discovered that this cloud of stars is actually a psrt of a lsrger object, a cumulus of stars we know as "galaxy". The "Milky Way" is a part of the galaxy where our solar system is, and hence we know this galaxy ("our" galaxy) as the "Milky Way Galaxy"

Now, the "disk" of our galaxy is a different thing. Our galaxy is of a kind known as "spiral galaxies", where the stars are joined in a series of spiral arms which they enter/exit in their orbit aorund the galaxy centre. Spiral galaxies look like a flat cake as the spiral arms are in a single plane, like the radius of a bike wheel. This plane were the spiral arms are is called the "disk" of the galaxy as it looks like a "disk" of matter surrounded by (apparently) nothing.

Now, as we lay in between of one of the spiral arms, when we look in the direction to the centre of the galaxy, we actually are looking across the "disk" of the Milky Way Galaxy and see the body of the spiral arm as a "cloud of stars" -what the anceint Greek called the "Milky Way".

(BTW, we don't have any actual image of our galaxy as a whole; the pixture you post is either a painting or a picture from some another spiral galaxy used to give a clue on how it looks "our" galaxy)

2) As you observe the night sky, why do stars apear to move westward across the sky?

I would say earth's rotation causes stars to appear moving westward around a central point (Polaris). And earth's revolution causes them to shift slightly to the west each night, over a period of a year. Am I right?

You got it. Good deduction! :wink:

As the stars are very far, they look like a "static" background against which we can perceive the relative movements of our planet...

3) "The earth is 4.6 billion years old. How many revolutions around the center of galaxy has it travelled? The sun is located in one of the spiral arms and completes one revolution in about 200 million years."

For the sun, I can calculate that it has completed 460000000/200000000=23 revolutions around the center of the galaxy? When the sun completes one revolution around the center of the galaxy, does the earth completes one revolution around this center, too?

Yes. The Earth travels along with the sun in its movement as they're part of a single gravitational system -so the Sun's gravity "drags" the Earth in its trip around the galactic centre.

Originally Posted by kingwinner

I have just started the space unit in school and have 3 questions. I hope someone can answer and explain. I would appreciate!

1) "If you look into the night sky, you will see a cloudlike band of stars that stretches across the sky. Because of its milky appearance, this part of the sky is called the "Milky Way". The "Milky Way" is the disk of the "Milky Way Galaxy".

I don't get this part! Are "Milky Way" and "Milky Way Galaxy" the same thing or are they different? What is a "disk" and what does it mean by "The 'Milky Way' is the disk of the 'Milky Way Galaxy'."?

Actually there is a misleading thing in the textbook there - the the "cloudlike band" is just a luminescence that appears to be behind the stars you can see. It required the invention of the telescope to realise that the luminescence in fact is the light of millions of previously unsuspected stars. Because it is a milky line or band traversing the sky like a road, it was named "The Milky Way".

The Greek word for milk was "galaxos" which is why, now we understand what the Milky Way is, we apply the word "galaxy" to other examples in the Universe.

Once it was realised that it was a conglomeration of stars and that there were other, incredibly distant, such conglomerations in the Universe as a whole, the term Galaxy was applied to them. It was natural therefore to refer to the particular conglomeration we are in, the "Milky Way Galaxy", after the "Milky Way", the band of luminescence which the ancients thought was a roadway for the Gods through the heavens.

For example, in the following picture, which part is the disk?

Think of it as a coin you're looking at from above. The conglomeration of stars you see there is not spherical, it is disk-shaped, ie 100,000 light years from one side to the other, but only 16,000 light years deep at it's deepest point in the centre, tapering away on either side - not unlike a flying saucer.

2) As you observe the night sky, why do stars apear to move westward across the sky?

I would say earth's rotation causes stars to appear moving westward around a central point (Polaris). And earth's revolution causes them to shift slightly to the west each night, over a period of a year. Am I right?

Yes. Astrology (not astronomy) is based on the movement of the stars across the sky during a year - though strictly speaking the zodiac is the Sun's apparent movement through the constellations, as the stars were always considered a "fixed background", and the Sun and certain stars (called "wanderers", or planetes) moved against that background, as of course did the moon. The great "paradigm shift" in the study of astronomy was to recognise that the Sun is in the centre of the solar system, and the Earth is travelling around it, rather than the other way around. The strength of the heliocentric theory was that it explained more naturally the recessional movements of the planets, without the use of Ptolemaic "epicycles". The planets appear to briefly move backwards at points in their forward motion against the star background, because during that time the Earth is on the same side as the Sun as them, and consequently is overtaking them.

3) "The earth is 4.6 billion years old. How many revolutions around the center of galaxy has it travelled? The sun is located in one of the spiral arms and completes one revolution in about 200 million years."

For the sun, I can calculate that it has completed 460000000/200000000=23 revolutions around the center of the galaxy? When the sun completes one revolution around the center of the galaxy, does the earth completes one revolution around this center, too?

Well, I hate to say this, but - obviously! The Earth's orbiting around the Sun means that gravity acts like a kind of chain which keeps the Earth within the confines of the Sun's neighbourhood. Clearly, then, whereever the Sun goes, the Earth goes likewise - as do the Moon and the other planets.

Your school textbook was really atrociously written, in my humble. Here's something slightly better written:
Just
Re-
-Member that you're standing on a planet that's evolving
And revolving at nine hundred miles an hour
That's orbiting at nineteen miles a second,
So it's reckoned,
The Sun that is the source of all our power.
The Sun and you and me,
And all the stars that we can see,
Are moving at a million miles a day
In an outer spiral arm at forty thousand miles and hour,
The Galaxy We Call the Milky Way.

The Galaxy itself contains a hundred billion stars,
It's one hundred thousand light years side to side.
It bulges in the middle, sixteen thousand light years thick
But out by us it's just three thousand light years wide.
We're thirty thousand light years
From Galactic Central Point,
We go round every two hundred million years,
And our galaxy is only one of millions of billions
In this amazing and expanding Universe.

The Universe itself keeps on expanding and expanding
In all of the directions it can whizz,
As fast as it can go, that's the Speed of Light, you know,
Twelve million miles a minute and that's the fastest speed there is.
So remember when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life
Somewhere up in Space,
'Cos there's bugger-all down here on Earth!
Eric Idle, Monty Python's Life The Universe and Everything (1983)

Thank you for explaining!