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| courses:phy100:8 [2023/08/22 01:45] – [3. Nebula] asad | courses:phy100:8 [2023/11/29 00:02] (current) – asad | ||
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| ====== 8. Nebulae and the Milky Way ====== | ====== 8. Nebulae and the Milky Way ====== | ||
| - | ===== - Our view of the Milky Way ===== | + | ===== - Milky Way ===== |
| + | ==== - Our view of the Galaxy | ||
| We are inside the disc of this galaxy, so we cannot see it from outside. But we can see different parts of the disk during different months of a year. | We are inside the disc of this galaxy, so we cannot see it from outside. But we can see different parts of the disk during different months of a year. | ||
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| - | ===== - We are not at the center ===== | ||
| - | How did we first realize that we are not at the center of our galaxy? In 1917, Harlow Shapley located a lot of globular clusters (tightly bound clusters of thousands of stars). And he saw that the globular clusters are distributed in a halo around a point which is almost 25,000 light years away from us. | ||
| - | {{: | + | How did we first realize that we are not at the center of our galaxy? In 1917, Harlow Shapley located a lot of globular clusters (tightly bound clusters of thousands of stars). And he saw that the globular clusters are distributed in a halo around a point which is almost 25,000 light years away from us. You can see the positions of these clusters in the side view below. |
| - | Here you see the positions | + | ==== - Structure |
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| - | {{https://upload.wikimedia.org/wikipedia/commons/thumb/1/ | + | The Milky Way has three main parts: a **disk**, a **bulge** and a **halo**. The disk is almost 100 kly (kilo/thousand light years) in diameter. The bulge is located at the center of the disk and the spherical halo surrounds the whole disk. The bulge is a little bit elongated like a //flattened football// and, hence, the Milky Way is a **barred spiral** galaxy. Spiral because the Galaxy has multiple **spiral arms** in the disk that spiral toward the Galactic Center (GC). The halo contains the globular clusters. |
| - | Our galaxy | + | Gravity would make the whole Galaxy collapse onto itself if not for the rotation of the stars and gas around the Galactic Center. Rotation opposes gravity and makes the galaxy |
| - | - The nuclear bulge | + | The Sun is **26 kly** away from the center, so it revolves around the center comparatively slowly. If you go closer to the center, your orbital speed will increase. As you go nearer, both the number of stars per cubic light-year and their average orbital speed increase. In the neighborhood of the Sun on average there are only 3 stars in 1000 cubic light-years, but near the core of our galaxy there are 10 million stars within just 1 cubic light-year. |
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| + | ===== - Stars and clusters ===== | ||
| + | ==== - Star populations ==== | ||
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| + | ==== - Star clusters and associations ==== | ||
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| + | ^ Type ^ Number of stars ^ Size (diameter) ^ Location ^ | ||
| + | | Open cluster | Tens to a few thousand | 14 -- 40 ly | Spiral arms and disk | | ||
| + | | Globular cluster | Lakhs to a million | 80 -- 320 ly | Halo and bulge | | ||
| + | | Associations | Five to seventy O/B type stars | 130 -- 650 ly | Spiral arm | | ||
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| + | ===== - Interstellar medium and traffic ===== | ||
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| ===== - Nebula ===== | ===== - Nebula ===== | ||
| Nebula is a cloud of gas and dust bright enough to emit radiation. But their mechanism of radiation is very different from the radiation of a star. Let us talk about the Orion Nebula and try to understand what exactly we are seeing in the picture. | Nebula is a cloud of gas and dust bright enough to emit radiation. But their mechanism of radiation is very different from the radiation of a star. Let us talk about the Orion Nebula and try to understand what exactly we are seeing in the picture. | ||
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| The position of the Orion Nebula is at lower part of the Orion Constellation in this picture taken using an ordinary camera. | The position of the Orion Nebula is at lower part of the Orion Constellation in this picture taken using an ordinary camera. | ||
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| Here is the image of the Orion Nebula taken by our telescope, Ashvin II. | Here is the image of the Orion Nebula taken by our telescope, Ashvin II. | ||
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| And here is the Orion Nebula through the eye of the great Hubble Space Telescope. Why is it so red? Yes, the colors are not real, and this is actually a composite image that combines visible-light and infrared radiations. But, it is still true that many nebulas look red. Why? | And here is the Orion Nebula through the eye of the great Hubble Space Telescope. Why is it so red? Yes, the colors are not real, and this is actually a composite image that combines visible-light and infrared radiations. But, it is still true that many nebulas look red. Why? | ||
| - | ==== - How a nebula shines ==== | ||
| Moons shine by reflecting the light of their parent stars. But nebulas shine by not reflecting but absorbing the light of their native and neighboring stars. Let me make the point more clear. | Moons shine by reflecting the light of their parent stars. But nebulas shine by not reflecting but absorbing the light of their native and neighboring stars. Let me make the point more clear. | ||
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| If the electron jumps to the second level, it emits a packet of light called the **H-alpha photon** and the wavelength of this light makes it red. Obviously then, inside a nebula there are a lot of 1-3-2 transitions go on giving rise to red H-alpha light. Do not ask me why it is called H-alpha. | If the electron jumps to the second level, it emits a packet of light called the **H-alpha photon** and the wavelength of this light makes it red. Obviously then, inside a nebula there are a lot of 1-3-2 transitions go on giving rise to red H-alpha light. Do not ask me why it is called H-alpha. | ||
| - | ===== - Nebula types ===== | + | ==== - Nebula types ==== |
| Nebulas that emit light via transitions of electrons are called **emission nebula**. They are pretty bright. Orion Nebula can be seen with naked eye as well if you go to a very dark location. Emission nebula can be red or green. Red light comes from hydrogen atoms and green light from oxygen atoms. | Nebulas that emit light via transitions of electrons are called **emission nebula**. They are pretty bright. Orion Nebula can be seen with naked eye as well if you go to a very dark location. Emission nebula can be red or green. Red light comes from hydrogen atoms and green light from oxygen atoms. | ||
courses/phy100/8.1692690349.txt.gz · Last modified: 2023/08/22 01:45 by asad