1. Option D Lesson 2: Astronomical Objects B.
Space, the final unit.
Option D Lesson 2: Astronomical Objects B.

2. Stars
We will look at stars in much more detail in another lesson, but in short:
A star is a massive ball of gas, pulled and held together by gravity with fusion going on at the centre.
There are a number of different “types of stars” and a number of ways in which stars exist… More to come later.

3. Binary Stars:
Binary Stars are pairs stars that rotate around a common centre.
More on these, why they are so useful and how we know about them later.

4. Moving away from the Solar System.
Stellar Clusters
Groups of stars that are physically close to each other, created by the collapse of the same gas cloud.
Approximately 100,000 ~3,000,000 (although some can have as few as a ~40) stars.
Usually in a region light years across.
Held together by gravity.
Two types of stellar clusters
Globular cluster – tight groups of hundreds of thousands of very old stars
Open cluster - contain less than a few hundred members, and are often very young– move in broadly same direction in space – referred to as stellar association or moving group

5. Galaxy
A galaxy is a collection of a very large number (a few million – hundreds of billions) of stars, gas and dust mutually attracting each other through the gravitation.
There approximately 100 billion galaxies in the observable universe.
There are three types of galaxies:
- Spiral (for example: Milky Way)
- Elliptical (for example: M49)
- Irregular (for example: Magellanic Clouds)

6. Spiral Galaxy. Large “central bulge” surrounded by a flatter disc.
Andromeda (the next closest galaxy to ours, about 3 million light years away…)
Large “central bulge” surrounded by a flatter disc.
Disc is made up of spiral arms that spread out from the central bulge.
There is evidence that at the centre of these galaxies is a super massive black hole.
Stars towards the centre of a spiral galaxy tend to be older than those in the arms.
Most observed galaxies are spiral galaxies.
 Galaxy M101
Galaxy ESO

7. Our Galaxy Spiral arm Nuclear Bulge Spiral arm Spiral arm
We belong to the Milky Way galaxy. The Milky Way is a spiral galaxy 1000,000 light years wide – 10,000 light years thick at the centre – has three distinct spiral arms .
The Sun is positioned in one of the spiral arms about two-thirds of the way from the galactic center, at a distance of about 30,000 light- years.
Average distance between the stars in our Galaxy is about 3.5 light years, but this doesn’t mean much since they are densely packed in the Nuclear bulge (around ly apart) and further apart in the arms.
Proxima Centauri (star closest to ours is about 4.5 light years away)
Spiral arm
Spiral arm
Nuclear Bulge
Disc

8. Spiral Galaxy.

9. Elliptical Galaxies
Elliptical galaxies are… elliptical in shape and do not have the spiral arms seen in Spiral Galaxies, they are more rounded.
Elliptical galaxies tend to be older than Spiral Galaxies and are dimmer.
Elliptical galaxies contain less dust and gas than spiral galaxies.
Elliptical galaxies are the most common galaxies, but are harder to identify as they are dimmer.

10. Elliptical Galaxy
Elliptical cross sections and no spiral arms.

11. Irregular Galaxies No “regular” shape… Small Magellanic Cloud
Hoag's Object, a ring galaxy.

12. Galaxy Clusters and Groups.
Galaxy Groups and Galaxy Clusters are large numbers of Galaxies held together by mutual gravitation.
Our local group (called the “local group”) contains about 50 Galaxies of which the Milky Way and the Andromeda Galaxy are the largest. It has a diameter of approximately 10 mega light years.
Galaxy Clusters are pretty much the same thing as groups, just larger, and are made up of hundreds (or even thousands) of Galaxies bound together by mutual gravitation.
Closest to us it the “Virgo cluster” about 5.9x107 Light years away
Galaxies in clusters are about 1024 m (or 108 light years) apart.
Cluster are grouped into “Superclusters”.
Superclusters are not bound by gravity!
Our galaxy is part of the Virgo Supercluster.
Clusters are separated from each other in an order of magnitude of about 1024 m (109 light years!) ( light years!!!!)

13. Nebulae
A Nebula is an interstellar cloud of dust, hydrogen gas and plasma. It is the first stage of a star's cycle..
Nebulae often form star-forming regions, such as in the Eagle Nebula.
We also use the term “Planetary Nebula” to refer to the remains of a dying (or dead) star.

14. Universe
Super-cluster (Virgo)
Group (Local Group)
Galaxy (Milky way)
Solar system
Inner planets
Earth
North America
Canada BC
North Van

15. Constellation
A constellation is a group of stars that APPEAR close to one another from Earth (they are not necessarily close to one another in the Universe!).
They can be made into patterns such as “The Big Dipper (or the Plow or in Hindu Sapta Rishi, meaning the "The Seven Great Sages)” and “Orion” …

16. Apparent motion of the stars.
Since the Earth is rotating (and orbiting the Sun), the stars (and constellations) appear to circle around the Earth.
The stars appear to be projected onto a spherical screen surrounding our earth called the “spherical sphere”
In the course of one night, it appears that the stars trace a circle around the “North” of our “celestial sphere”. With Polaris (the North Star) moving the least.
Over different seasons the constellations appear to cycle through the sky as we orbit the sun.

17. Apparent Motion of the stars.

18. Apparent Motion of the Stars
It’s worth noting that in the Southern Hemisphere, different constellations and Stars can be seen.
Southern Cross

19. The Celestial Sphere

20. The Celestial Sphere
Zenith = Point on the celestial sphere directly overhead
Nadir = Point on the c.s. directly underneath (not visible!)
Celestial equator = projection of Earth’s equator onto the c. s.
North celestial pole = projection of Earth’s north pole onto the c. s.

21. Stellar Distances.
You may have noticed that I have used different measurements of distance in this presentation.
We will have a whole lesson on stellar distances, but in summary:
1 AU (or Astronomical Unit) is the average distance between Earth and Sun
1Au=1.5 x 1011 m
1 Light year is the distance light travels in one year.
Since light travels at 3.0x108 m/s and in 1 year there are 60 seconds x 60minutes x 24 hours x 365 days.
1 ly = 9.46 x 1015m
Parsec (Pc).
Parsec stands for Parralax Arc Second. This is the distance to a star whose “Parallax is 1 arcsecond”. This is aproximately 3.09x1016m or 3.26 light years.