C.17 The Milky Way

When we try to pick out anything by itself, we find that it is tied to everything else in the

universe.John Muir (1838-1914)

For comparison of the following slides, the distance from the Sun to Neptune is 8.44 light hours or .000963 light years.

For comparison the Milky way is 100,000 light years in diameter.

Nebulas• A glowing cloud of gas

and dust is called a nebula

• There are 3 different kinds. Emmission, Dark and Reflection.

• Emission nebula - a cloud of glowing gas

• The Great Nebula in Orion is an Emission nebula.

• It is 1,344 light years away and 24 light years across.

The Trapezium. A group of 4 stars in the middle of the Orion Nebula. These are young, hot, blue stars.

Dark Nebulas

• In a dark nebula dust and gas that is not glowing is outlined by glowing gases and stars behind it.

• The Horsehead Nebula, which is close to the Great Nebula in Orion, is an example of a dark Nebula

• It is 13 light years across and 1500 light years away

The Snake Nebula in the constellation Ophiuchus

Barnard Dark Nebula in Ophiuchus

N44 superbubble - Larg Magellanic cloud

Reflection Nebula

• Reflecting Nebulas occur when clouds of gas and dust reflect the light of stars that are within.

• The Pleiades, Seven sisters, or Subaru is an example of a Reflection Nebula.

• It is about 12 light years in diameter and 440 light years away.

North American and Pelican Nebulas

The Heart and Soul nebulae stretch out nearly 580 light-years across, they are 7500 light years away in the constellation Cassiopeia

Cone Nebula

The Cone Nebula and the Christmas Tree cluster. The cone nebula is 7 light years long and 2,700 l.y. away.

Lagoon Nebula

Center of Lagoon NebulaThe light from M8 we see today left about 5000 years ago. Light takes about 50 light years to cross this section of M8.

Carina Nebula 2 light years in diameter and 8,000 l.y. away.

Hubble Space Telescope image showing Eta Carinae and the bipolar Homunculus Nebula which surrounds the star. The Homunculus was created in an eruption of Eta Carinae, the light from which reached Earth in 1843. Eta Carinae itself appears as the

white patch near the center of the image, where the two lobes of the Homunculus touch

Rosette Nebula

Coal Sack

Milky Way Structure

• Harlow Shapley deduced the structure of the Milky Way in 1920.

• He noticed that globular clusters were all in the same general area of the sky.

• In plotting their distances from the earth he noted that they form a spherical halo above and below the galactic plane.

• This halo was centered around a point thousands of light years away.

• Shapley realized that this point must be the galactic center.

Hypothetical Structure• Based on observations

people first felt that the Milky Way was a spiral galaxy and that we are located on the Orion spur.

• Our galaxy is about 100,000 light years across. We are about 33,000 light years from the nuclear bulge.

• Our sun and spiral arm rotates around the nuclear bulge in a clockwise direction.

Milky Way Structure cont.

• The use of different types of telescopes has helped us get a better idea of the shape of our galaxy.

• This is an infrared photo of our galactic disk and bulge taken by the COBE satellite.

Globular clusters - a halo of these exist around the Galactic core

Modern Galaxy maps

• The first good map of the Milky Way was made by Oort, Kerr and Westerhout (1958)

• They mapped neutral hydrogen in the plane of the galaxy.

• 1976 Yvonne and Yvon Georgelin mapped all of the bright nebula and showed they existed in the spiral arms.

Milky Way Map cont.

• In the 1990’s researchers first found evidence that our galaxy is a barred spiral galaxy.

• The early maps based on these observations looked like this one.

Studying the center• Observations of radio waves and infrared radiation help us penetrate the thick dust and light that hides the center.

• A very bright radio wave source is indicated in the center of our galaxy ( as bright as 80 million suns)

• In the center there is evidence of an extremely narrow radio source 10 A.U. across. This could be a massive black hole.

• It is called Sagittarius A• This slide is a radio image

of the central disk taken from the Very Large Array, near Soccoro, New Mexico.

Chandra X-ray picture of bulge

The Chandra X-ray Observatory is a satellite launched by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.

Chandra is sensitive to X-ray sources 100 times fainter than any previous X-ray telescope. The Earth's atmosphere absorbs the majority of X-rays. This requires a space-based telescope to make these observations.

Chandra Observatory is the 3rd of NASA's 4 great Observatories. The first was Hubble Space Telescope; second the Compton Gamma Ray Observatory, launched in 1991; and last is the Spitzer Space Telescope. Compton Gamma Ray Observatory Spitzer Space Telescope

The Spitzer Space Telescope is an infrared space observatory launched in 2003.The planned mission period was to be 2.5 years with an expectation that the mission could extend to five more years until the onboard liquid helium supply was exhausted. This occurred on 15 May 2009.

In keeping with NASA tradition, the telescope was renamed after successful demonstration of operation, on December 18, 2003. Unlike most telescopes which are named after famous deceased astronomers by a board of scientists, the name for SIRTF was obtained from a contest open to the general public.The contest led to the scope being named in honor of Lyman Spitzer, one of the 20th century's great scientists. Spitzer wrote a 1946 report for RAND describing the advantages of an extra-terrestrial observatory and how it could be realized with available (or upcoming) technology.

The Compton Gamma Ray Observatory was the second of the NASA "Great Observatories" to be launched to space, following the Hubble Space Telescope. It was named after Dr. Arthur Holly Compton (Washington University in St. Louis), Nobel prize winner, for work involved with gamma ray physics. The observatory was launched on the Space Shuttle Atlantis, on 5 April 1991 and operated until its deorbit on 4 June 2000. It was deployed in low earth orbit at 450 km (280 miles) to avoid the Van Allen radiation belt. It was the heaviest astrophysical payload ever flown at that time at 17,000 kilograms (37,000 lb)

Bulge x-ray info• The previous slide

was a mosaic of images taken by the x-ray satellite Chandra.

• It showed several large sources of x-rays which are believed to by neutron stars and a strong candidate for a black hole in the central bright spot of the slide.

Spiral Shape• People deduce that the

galaxy has a spiral shape.

• Open clusters always appear in spiral arms.

• Spiral arms are where young stars are found.

• Emission nebula are found in the spiral arms.

• Plotted distances to these regions trace out parts of 3 spiral arms.

Intersteller medium• The IRAS satellite,

pictured here, has discovered vast clouds of Hydrogen gas between and connecting with the spiral arms.

• This satellite takes infrared pictures and can detect matter that is barely above absolute zero.

IRAS• The Infrared Astronomical Satellite (IRAS) was the first-ever space-based observatory to perform a survey of the entire sky at infrared wavelengths.

• Launched on January 25, 1983, its mission lasted ten months.

• The telescope was a joint project of the United States (NASA), the Netherlands (NIVR), and the United Kingdom (SERC).

• IRAS's life was limited by its cooling system.

• The on-board supply of fluid helium was depleted after 10 months on November 22, 1983.

• This caused the telescope temperature to rise, preventing further observations

Molecular clouds• Vast clouds of gas

and dust exist in and around our galaxy.

• These clouds like the Orion cloud pictured here, form the building blocks of stars in our galaxy.

The Local group• Our galaxy is the 2nd largest of a collection of galaxies called the local group.

• Included in this group are the Large and Small Magellenic clouds, Sculptor and Fornax dwarf galaxies, Leo I and Leo II dwarf galaxies, M33, elliptical galaxy Maffei I and Maffei II and of course M31 the giant Andromeda galaxy.

M33 – Triangulum galaxy or pinwheel galaxy3rd largest of the local group, 3,000.000 l.y. away

Magellanic Stream• A large cloud of

hydrogen gas surrounds our galaxy and includes the Large and Small Magellanic clouds.

• This cloud orbits our galaxy as do the Large and Small Magellanic clouds.

Large Magellanic cloudThe brightest galaxy visible from Earth. It is an irregular small galaxy.It is 160,000 l.y. away. It is the 4th largest galaxy in the local group.The Tarantula nebula is the largest nebula known.

Large Magellanic Cloud

Tarantula Nebula

• The Tarantula Nebula in the Large Magellanic Cloud is largest Nebula known.

• With a diameter of 800 l.y. and a mass of 300,000 suns, it dominates the Large Magellanic cloud

• If it were as close to us as the Great Nebula in Orion it would be as large and as bright as the moon.

Hubble Mosiac of Tarantula Nebula (30 Doradus, Central Region The image of 30 Doradus is a mosiac of 5 fields-of-view of one of the Hubble Space Telescope's instruments, the Wide Field Planetary Camera-2. Three filters were used creating an image that shows the stars according to their real colors.

Small Magellanic Cloud.200,000 l.y. away. May have been warped in shape by the Milky Way.7,000 l.y. across and contains several million stars.

A view of the Large and Small Magellanic clouds from the

southern skies

M31 Andromeda Galaxy, 2.2 million l.y. away rotating counterclockwise and formed at the same time as ours.

Scultpor dwarf galaxy

• About 8,000 l.y. in diameter.

• Has about 2 million stars like our sun.

• Very loosely organized galaxy.

• It orbits the Milky Way along with a number of other dwarf galaxies.

Fornax dwarf galaxy

Sextans dwarf galaxy

Leo I dwarf galaxy

Sagittarius dwarf galaxy

• The closest galaxy to the Milky Way is not the Large and Small Magellanic clouds.

• Named the " Sagittarius Dwarf ", this small galaxy was discovered in 1994.

• The reason the Sagittarius Dwarf hadn't been discovered earlier is because it is dim, it is spread out over the sky, and there are so many Milky Way stars in front of it.

• The distance to the Sagittarius Dwarf was measured to be about one third of the distance to the LMC.

• Astronomers believe that this galaxy is slowly being torn apart by the gravitational forces of our Galaxy.