Scale

For an overview of astronomy, this video gives a pretty good sense of time and scale in our universe.

Inventory of the Universe

The universe consists of the following objects:
- Stars, which are organized into galaxies.
- Planets, which orbit stars.
- Moons, which orbit planets.
- Debris, in the form of comets, asteroids, meteoroids.
- Interstellar gas and dust.
- Exotic objects, such as black holes, neutron stars, quasars, etc.

Our Cosmic Address

There are billions of billions of stars out there. Our Cosmic Address among these is as follows:
- Our Solar System consists of our Sun and objects which orbit it, including the 8 planets.
- Our Solar System is one among more than 100 billion star systems in the Milky Way Galaxy.
- The Milky Way and the Andromeda Galaxy are the two largest out of about 40 galaxies in the Local Group.
- The Local Group lies near the outskirts of the Local Supercluster.
- The local Supercluster fades into the background of structure throughout the universe.
- Altogether, there are about 100 billion galaxies in the visible universe.
This video shows the broad range of sizes in the Cosmos, with some cool music in the background.
Powers of 10: This interesting applet also explores size scales, but with a focus on powers of 10,
For fans of Neil deGrasse Tyson, this video discusses our cosmic address.

Useful astronomical distance scales

A useful unit of distance in our Solar System is the AU (astronomical unit), defined as the average distance of Earth from the Sun.
- This distance is about 150 million km.
- In light-time, this distance equals 8.3 light-minutes.
Pluto is about 5 light-hours or 40 AU's away from the Sun.
The nearest star, Proxima Centauri, is 4.3 light-years away.
The Milky Way is about 100,000 light-years across and 1,000 light-years thick.
We are located about 2/3rds of the distance from center to edge or about 30,000 light-years.
In any direction, the edge of the visible universe is about 14 billion light-years away.
For the various techniques of measuring distance, see astrometry.

Example	Light Time	Approximate Distance
~Earth-Moon	1.2 seconds	384,000 km
~Sun-Mercury	3 minutes	54,000,000 km
Sun to Earth (1 AU)	8.3 minutes	149,600,000 km
~1.5 x Sun-Jupiter Distance	1 hour	6.7 AU
Sun-Pluto distance	5 hours	40 AU
Voyager-1 (April, 2001)	11 hours	80 AU
Light-year	1 year	63,000 AU
Proxima Centauri	4 years	253,000 AU
Betelgeuse	470 years	30 million AU
Most naked-eye stars	Several thousand years	~100 million AU
Size of Milky Way	100,000 years	6 billion AU
Andromeda Galaxy	2.2 million years	140 billion AU

Speed and distance

A jet moves about 1 million times slower than light; a rocket moves about 27 thousand times slower.
At rocket speed, it would take about 100,000 years for us to travel to the nearest star (Proxima Centauri).

Destination	Jet (600 mi/hr=0.9 μc)	Rocket (25,000 mi/hr=37 μc)	Sunbeam (186,000 mi/sec)
Moon	16.5 days	9.4 hr	1.2 sec
Sun	17 years 8 months	4 months	8.3 min
Mercury	10 years 10 months	3 months	5 min
Venus	5 years 5 months	1.5 months	2.5 min
Mars	8 years 10 months	2.5 months	4 min
Jupiter	74 years 3 months	1 year 9 months	35 min
Saturn	150 years 5 months	3 years 7 months	1 hr 11 min
Uranus	318 years 6 months	7 years 7 months	2 hr 30 min
Pluto	690 years 1 month	16 years 5 months	5 hr 25 min
Alpha Centauri	4.8 million years	114,155.2 years	4.2 years
Sirius	9.6 million years	228,310.4 years	8.4 years
Center of Milky Way	30 million years	800,000 years	38,000 years
Andromeda Galaxy	2 trillion years	50 billion years	2.2 million years

Measuring distance

Several techniques have been developed to measure cosmic distance, and different techniques must be used on different scales. This is the brief version. For more details, visit this wikipedia article.

method	not effective beyond this distance
radar ranging	1 AU
stellar parallax	1000 light-years
spectroscopic parallax	50,000 light-years
variable stars	100 million light-years

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