Friday 11 March 2011

************ 2ND EXAM= FRIDAY 25 MARCH 2011**************

The second inclass exam, on Stars, will be Friday of the week after spring break. Here is an old exam that covers the same material. I suggest you look at this carefully. Sara and Henry will go over this exam on the Tuesday and Thursday before the test. MAKE SURE you look at the last page and note the questions based on the diagrams. You will see similar questions on the test.

Old star exam, page 1

Old star exam, page 2

Old star exam, page 3

Old star exam, page 4

Old star exam, page 5

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Although we often think about space as a vacuum, that is not true. There are gas and dust particles between the stars. The material between the stars is called the interstellar medium (ISM). In most of space, the gas atoms and molecules and dust particles are spread VERY thinly compared to the air in this room. However, there are "clumps" of gas and dust some places. Astronomers use the word "nebula" to refer to any cloud of gas and dust in space. There are 3 different basic types of nebula based on 3 different types of physical process: (1) reflection (2) emission (or HII) and (3) dark. Many nebula combine all three types and are very complicated regions.

Pleiades reflection nebula

The Pleaides are a star cluster, a large number of stars born at about the same time in the same place. The brightest 6 or 7 stars are easily visible to the naked eye as a small "dipper". They are also known as the "Seven Sisters". The stars are surrounded by some interstellar dust. Starlight reflected off the dust makes the dust visible- this is thus a reflection nebula. The brightest stars in the Pleiades are hotter and more luminous than the Sun, but are *not* hot enough to excite the gas atoms mixed with the dust to glow like a neon sign, so the light we see is simply reflected starlight.

The crossed lines from the brighter stars, as well as the perfectly circular "halos" around the stars are internal reflections in the camera, not real parts of the stars.

Orion (1)

Orion (2) Two wide angle views of the constellation Orion. The first points out two bright stars of particular interest- Betelgeuse, a very luminous red supergiant star, and Rigel, a luminous and hot B type star.

Below the 3 bright "belt" stars is the Orion Nebula, which looks like a fuzzy patch of light, and is a region of current star formation. In the second image, a photograph taken with a wide angle telescope, you can also see other nebula around Orion- the whole Orion region is a complex assortment of nebula of all types.

Center of Orion Nebula These images are of the central regions of the Orion Nebula. The image on the left is taken in visible light, and shows a mixture of dust and glowing gas. This part of the Orion Nebula is an emission nebula, or HII region (ionized hydrogen region). The electrons in the atoms are excited by the ultraviolet light from hot stars in the nebula and glow, something like a neon light. (But the electrons in the HII regions are excited by high energy photons, rather than by moving electrons as is the case of a neon light or the aurora).

The image on the right is taken in infrared EMR. The infrared EMR penetrates the dust much easier than the visible light, because of the longer wavelength of the infrared radiation, so the infrared images reveals many more stars. Many of these stars are newly formed. The Orion Nebula is often called a "stellar nursery".

Horsehead and Flame Nebulae

Horsehead cartoon

The famous "Horsehead Nebula" is a good example of a dark nebula, a huge volume of dust which blocks the light from regions behind it. The dust would be very spread out, but the volumes are so enormous that the dark nebula can block most of the background light. On the left, the orangish looking area is a nebula known as the "Flame Nebula". This is an ionized hydrogen cloud, or HII region, in which the atoms are excited to glow by a very hot star, much like the Orion Nebula.

With a little imagination, you can get a feel for the 3-dimensional structure of the volume of space depicted in this image. In the region "above" the Horsehead, space is less dusty than the region below it, so one can see stars past the dark nebula (background stars) as well as stars between us and the dark nebula (foreground stars). "Below" the Horsehead, there are far fewer star visible, as the dark nebula hides background stars, while foreground stars are still visible.

Eagle Nebula / Pillars of Creation (1) .

Pillars of Creation (2) .

Pillars of Creation (3). The first image is taken with a groundbased telescope of the Eagle Nebula, a region of recent star formation. The other spectacular images, taken with the Hubble Space Telescope, are a small part of the Eagle Nebula. These columns of dust and gas are popularly known as the "Pillars of Creation". Small knots seen in the pillars are protostars, regions of gas and dust that are being compressed by gravity. These protostars will become true stars once their cores become hot enough for fusion to commence.

Interstellar Ethyl Alcohol In cooler regions of vast dark nebulae a wide variety of different molecules have been detected, some of surprising complexity. This paper announces the discovery, in one of these dark nebulae, of molecules of ethyl alcohol, a favorite molecule of many college students. These molecules are detected from their emission lines, usually in the microwave or radio region of the spectrum.

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Reading for Exam 2 in Horizons 12th edition:

Chapter 6: read it all

Chapter 7:

section 7-1 read 1st half page on p. 123 (Composition of Sun)- skip rest of 7-1

section 7-2 read stuff on pages 126-127, skip rest

section 7-3- read it- skip section called "Nuclear Binding Energy" (p. 131). Read p. 132-133 (stuff associated with section 7-2).

Chapter 8:

section 8-1 - read it

section 8-2 - skip section "Absolute Visual Magnitude"- read rest

section 8-3 - skip section "The Balmer Thermometer"- read rest

section 8-4 - read up to "Luminosity Spectral Classification" on p. 154. Do look at Figure 8-11 (You will see an HR diagram on the exam!)

section 8-5- read it

section 8-6 - read at least p 164-165 - look at Fig 8-20

Chapter 9:

section 9-1 - read it

section 9-2- read up to "Contagious Star Formation"

section 9-3 - skip it

section 9-4- read it

section 9-5 - read it

Chapter 10:

section 10-1 - skip pages 202,203, 204 (but do look at Figure 10-5)

section 10-2- read all

section 10-3- skip entire section

section 10-4- read all

Chapter 11:

section 11-1 - read up to "The Evolution of Pulsars" on p 229. Look at p 230-231 and Fig 11-3. Skip rest of section 11-1.

section 11-2 and 11-3 - skip - we will talk about black holes later