Galaxies Stellar Neighborhoods. What are Galaxies? Galaxies are –Huge –Held together by gravity –Comprised of… Stars Objects that orbit those stars The

Galaxies

Stellar Neighborhoods

What are Galaxies?

• Galaxies are– Huge– Held together by gravity– Comprised of…

• Stars

• Objects that orbit those stars

• The left over gas and dust from the formation of those objects.

What are Galaxies? (cont)

• Galaxies rotate.– Stars orbit the center of galaxies at speeds ranging from

210-240km/s (470,000-540,000mph)

• Galaxies are where the matter is.– Areas in between galaxies have a density of less than

one atom per cubic meter.

– The air we breath contains 5,000,000,000,000,000,000,000,000 atoms per cubic meter. (5x1024 or 5 septillion)

How many galaxies?

• Scientists can estimate that there are about 100,000,000,000 galaxies (1x1011 or 100 billion)

How Did Galaxies Form

• Galaxies formed as rotating clouds of gas, compressed under gravity, began forming stars.

• The oldest galaxies scientist can see formed 750 million years after the big bang.

How Big Are Galaxies?

• Galaxies have anywhere between 10 million and 1 trillion stars.

• Range in size from 27,000,000,000,000km to 3,000,000,000,000,000km

• Expressing distances in space is difficult when we used dimensions created for measuring Earth.

Light Year

• All light travels the same speed in a vacuum and is the fastest known attainable speed.

• 3x108 m/s• A Light Year is the distanced traveled by

light over the period of one year.• 9,460,000,000,000 km• Most galaxies are between 3,000 and

330,000 light years across.

Structure of Galaxies

• Most galaxies have two main parts– Central bulge– Surrounding Halo

• Similar in shape to the pogo-ball• Galactic Bulge

– Most dense region of galaxy – Located in center of galaxy– Contains super massive black hole

• Halo– Thinner, outer region of galaxy

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Types of Galaxies

• Galaxies are grouped according to their shapes.– Elliptical– Spiral– Peculiar



Elliptical

• An ellipse is like a specific kind of stretched circle.

• Elliptical galaxies are like spheres that have been pulled to make them the shape of a football.

• Stars within elliptical galaxies do not rotate all together but individually around the center.

Types of Elliptical Galaxies

• Elliptical galaxies are categorized based on how “round” they are on a scale of 0-7.– E0 galaxies are nearly spherical– E7 galaxies are the most elliptical



Elliptical Galaxies (cont)

• Elliptical galaxies tend to be some of the largest galaxies.

• They also contain mostly older, more evolved stars.

• The haphazard rotation of stars within an elliptical galaxy suggest that these galaxies formed as a result of interactions between galaxies, such as a collision and merger.



Spiral Galaxies

• Spiral galaxies are disks of stars all rotating along the same plane around a central point.

• Spiral galaxies tend to have pronounced galactic bulges.

• Halo has been separated into areas of high and low density.– Areas of high density are called Arms.



Types of Spiral Galaxies

• Spiral galaxies are categorized based on the tightness and definition of the arms and the size of the galactic bulge.– Sa tightly wound, poorly defined arms, large

galactic bulge– Sb intermediate – Sc Open, well defined arms, small galactic

bulge.

Barred Spiral Galaxies

• Most spiral galaxies have a band of stars that extends outward to either side of the core, and are called barred spiral galaxies

• Formed from tidal interaction with other galaxies

• Have same categorization system as normal spiral galaxies. SBa, SBb, SBc

Peculiar Galaxies

• Peculiar galaxies develop unusual shapes due to interactions with other galaxies– Ring galaxies– Lenticular galaxies– Irregular galaxies– Dwarf galaxies

Ring Galaxies







Lenticular• Intermediate between spiral and elliptical

galaxies.

• Can be barred or un-barred (SB0 and S0)

Irregular Galaxies

• Some galaxies do not fall into any reasonable category– Irregular-I have some structure

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Dwarf Galaxies

• Small galaxies containing up to several billion stars.

• Most of the galaxies in the Universe

• Can be classified as being elliptical, spiral, or peculiar.

Our Galaxy

• Milky Way – The word galaxy is derived from the Greek

word for milk

• Visible from Earth as a band of light in the night sky.

• Large Barred Spiral Galaxy

How Big?

• About 100,000 light years across and about 1000 light years thick

• Contains between 200 and 400 billion stars

• Has a total mass of more than 600 billion times the mass of our sun.

• Difficult to look at as we are inside it.

You are here…

• It is believed that our galaxy contains 4 major arms and at least two minor arms.

• We are located on a smaller arm named Orion’s arm.– 24,000 light years from the center

– We take 225-250 million years to orbit the galaxy.

– Since humans existed on earth we have traveled less than 1/1000th of that journey.



We are not alone…

• Galaxies that are near one another are categorized as Groups.

• Contain fewer than 50 galaxies• Encompass an area 3-6 million light years in

diameter.• Milky Way is part of the Local Group of galaxies

– Also contains the Andromeda Galaxy



Clusters

• Larger than groups, however there is no sharp dividing line.

• Contain between 50-1000 galaxies within an area of 6-60 million light years.

• We are not part of a cluster of galaxies

Superclusters

• Largest organization of galaxies• Can span several hundred million light

years.• There are approximately 10 million

Superclusters with an average of 10,000 galaxies in each.

• We are located in the Local Super Cluster or the Virgo Super Cluster



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Galaxies Stellar Neighborhoods. What are Galaxies? Galaxies are –Huge –Held together by gravity –Comprised of… Stars Objects that orbit those stars The