1. Introducing the Stars

2. There’s No Shortage!
Let’s begin by glancing around the sky, to see what kinds of stellar vistas we might encounter here and there.

3. Looking Around (Virtually)
Open the Starry Night College 7 simulation software. (Note: this will later be a tool in at least one of the assignments to be submitted. Take this opportunity to learn how to use it. Explore its potential and its many neat features!) You will be taken to a ‘home’ start page (for me, Kingston) at the current time of day, and on today’s date. I get the picture shown above – you can select a panorama that suits your taste (lakes; mountains; Antarctica; Stonehenge…)

4. Some Convenient Simplifications
By design, the Starry Night software simulates the sky as it should actually appear. For the moment, however, we will do two unusual things (not available to real astronomers!):
Thanks to the rotation of the Earth, stars are sometimes below the horizon, and thus invisible. So we will look under the drop-down ‘View’ tab and hit “Hide Horizon.” The Earth becomes transparent, so we can look in any direction and see any star in the sky!
So that we are not bothered by scattered sunlight, which makes the stars invisible in the bright daytime sky, we will look under the drop-down ‘View’ tab again and hit “Hide Daylight.”

5. Finding Fields of Interest
In Starry Night, there are two ways of finding an object of interest – say, Polaris (the North Star):
search for it by name, using the ‘search bar’ on the upper right ; or
enter its coordinates (using one of the mapping systems used by astronomers) into the “Center on” command in the ‘View’ drop-down list.
Once you are centered on the target, you can zoom in to various extents, using the – + symbols to the lower left. This allows you to simulate what you would see if just gazing up by eye, or if instead you use devices ranging from simple binoculars to extremely powerful telescopes.

6. First Example
Using the ‘search’ tool to find Polaris, and go to it. Do you see the Big Dipper? In the picture above, it is to the left of Polaris. Its location will depend on the time of day at which you do this: thanks to the Earth’s spin, the stellar backdrop rotates around once a day. (To see this, go to the time shown at the upper left of the Starry Night window and advance the clock hour by hour.) The main thing to note here is that we see some hundreds of stars sprinkled at random over the field of view. There are plenty out there to study! If you zoom in (using the + sign at the lower left), you will see many more – in effect, you are looking through powerful telescopes, bringing fainter stars into view.

7. Exercises Using the Starry Night software, now do the following:
Search for Mizar. What constellation is it in? Zoom in until the field of view is about 20 degrees across. This is a double star that sharp eyes can pick out in the real sky at night – try it!
Search for the ‘Milky Way Centre’ (which is best seen from the southern hemisphere) and zoom in until the field is about 1 degree across. Note how dense and rich this field of stars is!
Search for the Pleiades, and zoom until the field is about 3 degrees across. This is an open star cluster that can be seen with the unaided eye in winter, in the constellation Taurus. Sharp eyes can pick out 7 or so stars (hence the name “the Seven Sisters”)
Search for M13, and zoom until the field is about 1 degree across (about twice the diameter of the full moon). This is a globular star cluster.

8. What Have We Discovered?
We have seen that stars are found in various arrangements:
some apparently single and widely separated;
some in close pairs;
some in very rich, densely-packed fields;
others in clusters of different sorts.
We will start by considering single stars in the vastness of space.

9. What We Would Like to Know [basic physical attributes]
Distances (Critical for many purposes!)
Luminosities
Masses
Sizes/shapes
Densities (Hints of structure.)
Temperature
Composition
Internal Structure

10. … and Yet More
Correlations among these! (For instance, does the brightness of a star depend on its mass?) This is how science is done!
Ages
Sources of energy (Are all stars like the sun?)
Potential longevity with various fuels
Stellar variability and activity
Stellar formation
Stellar death

11. First, Though: Why Is the Sun So Very Bright?
Perhaps it is a typical star, like many others - just particularly nearby, hence extraordinarily bright in appearance.
Or perhaps it is fundamentally different from the stars – bigger, brighter, special, maybe of an utterly different nature or composition.

12. But Let’s Turn the Argument Around
Assume the sun is an average star! This implies that the other stars look faint because of their remoteness. If we knew just how much fainter they are, we could estimate their distances.
Surprisingly, we can see only a few thousand stars at night! Even collectively, they provide only very feeble illumination. Clearly, they are much less than 1/1000 as bright as the Sun.

13. A Clever Idea: Chinks of Light
The ‘average star’ looks about one trillionth as bright as the sun
That’s 1 / 1,000,000,000, (or 1 / 1012)
In other words, if there were a million million visible stars, the night sky would be as bright as day.

14. So How Far Away Are the Stars?
Since brightness falls off like
the square of the distance,
and
one trillion = one million x one million = (one million)2
we deduce that the average visible star must be about
one million times as far away as the sun – a distance of about 15 light years. In fact, that’s roughly correct. [The sun actually is a run-of-the-mill star!]

15. Some Real Numbers
The nearest known star is ~4 light years away. The bright star Sirius is ~9 light years away. This means that the light reaching us now from the stars left them some years ago. Astronomers are always looking ‘into the past.’ For nearby stars, this is not enough of a ‘time lag’ to make any difference in our studies. (But for remote galaxies, and in cosmological studies, it matters!)

16. That’s Amazingly Remote!
The nearest star is four hundred million times as far away as the moon!
Think about the implications for human travel to the stars.

17. Are Stars ‘Close Neighbours’? How Big is the Solar System?
Distance to Neptune ~ 30 A.U. = 4.5 billion km
Let’s call this the size of the Solar System)
Within this distance there
is a busy swarm of planets,
moons, asteroids, etc.
(Please remember, though,
there is also a big surrounding
‘halo’ of tiny objects in the
Oort Cloud)

18. So… At a distance of 4.2 light years (just over 40 trillion km) from
us, even the very nearest star
is ~ 11,000 “solar system sizes” away.
Interstellar space is predominantly empty (and it is even more so between the galaxies). We live in very atypical surroundings
on a rock
very close to a star

19. On the Other Hand, Stars Move…
Reopen Starry Night, and search for a star with the boring ‘catalog’ name’ HIP (also known as Cyg 61B). Center on it, and zoom in until the field of view is about 1 degree across. You will see two relatively bright stars near each other.
Now go up to the Date shown on the upper left, and click on the year (2017, as I write these words). Use the up arrow key on your keyboard to advance this one year at a time. What you notice is that this star is drifting slowly across the field of view! (All stars move through space; this one is close enough to us for the effects to be measurable by astronomers, although still slow.)
Question: Do stars ever come physically close to each other? Interact? Perhaps even collide?

20. If So, How Often? Our Sun is typical, moving at
about 30 km/sec relative to
its stellar neighbours.
At this speed, it would take about one trillion seconds (30,000 years) to cover the distance to the nearest star.
Does this mean that the sun comes close to other stars as frequently as that? Were we close to another star even as recently as some tens of thousands of years ago?

21. Could the Solar System Even Have Formed through a Near-Collision of Two Stars?
We considered (and dismissed) this possibility in ASTR 101. The problem is that the stars move individually in random directions.

22. Analogy: Do Bullets Collide?
Yes – but very rarely!

23. Conclusion
Collisions, or even close passages, between stars are very rare indeed. (This is good for life on Earth!)
But the stars don’t completely ignore each other in the way that speeding bullets do
Instead, each star moves under the combined gravitational influence of all the material in the Milky Way galaxy – the other stars, gas, dust and dark matter.

24. Moreover, Star Clusters Exist (in these close confines, stars can interact more directly)

25. There are Even ‘Close Binaries’ [Here an artist’s impression, not a real photo]
These are stars
literally in contact,
in a mutual
gravitational orbit.
(Much more on binary
stars later.)