The Solar System

Planets: the larger bodies orbiting the Sun (or any other star)

The planets of our Solar System [& Pluto]

The biggest planet is….-- but the biggest object in the Solar System, by far, is …..

Uranus with 5 moons

A moon (a.k.a. satellite): an object of any size orbiting a planet

Our star – the Sun

T/F: Our Sun is a star.

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Star: Large, spherical ball of gas…...

(…that’s just a partial definition)

Surface of Sun – Gas bubbles

Solar Flare – an explosive event on the Sun’s surface

A bit of the sky, showing stars

If the Sun is a star, why does it look so big compared to allof those

other stars?

Star

Large, spherical ball of gas that produces its own energy by nuclear fusion.

Nuclear fusion

Atomic nuclei combine

to form larger nuclei,

releasing energy in the process

The Sun’s fusion reaction is calledHydrogen Fusion:

H+H+H+H He + energy (heat) [ interpretation: 4 atoms of Hydrogen(H) combine to produce

one atom of Helium(He), plus a little bit of heat ]

The “fuel” for this reaction is atoms of hydrogen;the “products” are helium atoms plus heat (energy)

Stars commonly appear clumped together as star clusters -- gravitationally-bound groups of stars, each one containing

hundreds to thousands of stars

Pleiades, a.k.a. “Seven Sisters”- a nearby star cluster

Stars, star clusters, etc. collect into much larger groups called galaxies.

The Sun is a star in the Milky Way Galaxy.

The Milky Way Galaxy is a so-called spiral galaxy. This picture of a different spiral galaxy, one that is seen face-on.

All of the single stars (points of light) seen in this photo are stars that are in our own Milky Way Galaxy

A spiral Galaxy seen Edge-on-the actual 3D-shaped is like a pancake with a lump in the center

How big is a galaxy?

# of stars: about a billion or so (smallest)- up to 100’s of billions for

large galaxies

Diameter: Very large: what units of distance should be used?

-- using miles gives awkwardly numbers.

-- a larger unit is needed.

-- for example…..

Light Year --is based on the “speed of light”: 186,000 miles/sec

Defn: 1 Light Year (1 LY) = the distance that light travels in one year.

*** - even YOU can figure it out! ***

Compute 1 LY from the (well-known??) equation:

“distance = velocity x time”

distance = speed of light x time (one year)

1 LY = (186,000 mi/sec) x (~31 million sec/yr)

= 5.86 Trillion miles ( roughly 6 trillion)

( = 5,860,000,000,000 miles )

Keep this point in mind about the connection between light-travel-time and distances:

** It takes light one year to travel a distance of one light-year **

Try to make all 3 columns of the table below make good sense to you.

Light-Travel-Time from the Earth

TO TIME DISTANCEa. the moon 1.25 sec 1.25 light-secondsb. the sun 8.5 minutes 8.5 light-minutesc. Pluto ~6 hoursd. Alpha Cen 4.3 yrs 4.3 LYe. North Star 650 years 650 LYf. M31 2.5 million yrs 2.5 M. LY

..so just keep in mind that all of those units of length are based on the speed of light;

- for example: 1 light-minute = the distance that light travels in one minute

The “Milky Way” galaxy is a

spiral galaxy.

The Sun is in a spiral arm half-way out

from the center.

Map of Milky WayOur Sun(yellow dot) is in a spiral arm, ½-way out

Galaxies contain more than just stars. The irregular-looking blobs are either hot (pink) or cold (dark) interstellar clouds

Galaxies occur in galaxy clusters.This is M31 (the Andromeda Galaxy)…

…. and friends (2 smaller neighboring galaxies)- sort of a “triple galaxy”

The “Local Group” is the name of the (small) cluster of galaxies to which the Milky Way belongs. The map above shows about 2/3 of its members (fewer than 40, total).

The galaxies closest to the Milky WayThe two Magellanic Clouds(LMC & SMC) are easily visible to the naked eye.

LMC

SMC

comet

A “nearby” galaxy cluster: Distance = 280 Million LY

Distance = 500 million LY

Distance over 5 BILLION LY. The galaxies look progressively smaller as we look at more distance clusters simply because of the increased distance. The

largest galaxies in large clusters are all about the same size.

This is a map of

superclusters out to

1 billion LY from the

Milky Way (which is at the center)

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Supercluster: – a cluster of

galaxy clusters

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# of galaxies within 1 B ly > 60 million;

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# of stars ~ 250,000 trillion

What can you see with the naked eye? [outside of the Solar System]

- Milky Way stars! (meaning only stars in our own galaxy)i.e., you cannot see any individual stars in any other

galaxy;-- they’re just too far and too faint

- 3 other galaxies, ALL in the Local Group:M31, LMC, SMC

Note: Every star visible to your eye is part of the Milky Way!

• Numbers to Know• Distance to nearest star ---------------- 4.3 Light Years

– ( typical distance between stars )

• Size of Milky Way Galaxy (a typical “big” galaxy):• Number of stars -------- 400 Billion• Diameter -------- 100,000 Light Years

• # of stars in ….• our Solar System ------------- 1• a star cluster ------------ 100’s to 1000’s• a galaxy ------------ Billions: ( ~ a billion to

~ a trillion

Heirarchical structure of the Universe from largest to smallest

The Universe: includes all objects below plus all of the space in between

Supercluster largest; most stars

Galaxy cluster

galaxy

star cluster planetary system (e.g., Solar System) one star per system

stars

planets

moons smallest (diameter)