APOD: North Star: Polaris and Surrounding Dust (2023 Apr 11)

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APOD: North Star: Polaris and Surrounding Dust (2023 Apr 11)

Post by APOD Robot » Tue Apr 11, 2023 4:06 am

Image North Star: Polaris and Surrounding Dust

Explanation: Why is Polaris called the North Star? First, Polaris is the nearest bright star toward the north spin axis of the Earth. Therefore, as the Earth turns, stars appear to revolve around Polaris, but Polaris itself always stays in the same northerly direction -- making it the North Star. Since no bright star is near the south spin axis of the Earth, there is currently no bright South Star. Thousands of years ago, Earth's spin axis pointed in a slightly different direction so that Vega was the North Star. Although Polaris is not the brightest star on the sky, it is easily located because it is nearly aligned with two stars in the cup of the Big Dipper. Polaris is near the center of the eight-degree wide featured image, a digital composite of hundreds of exposures that brings out faint gas and dust of the Integrated Flux Nebula (IFN) all over the frame as well as the globular star cluster NGC 188 on the far left. The surface of Cepheid Polaris slowly pulsates, causing the famous star to change its brightness by a few percent over the course of a few days.

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Ann
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Re: APOD: North Star: Polaris and Surrounding Dust (2023 Apr 11)

Post by Ann » Tue Apr 11, 2023 6:52 am


Nice APOD and helpful links! :D

However...

APOD Robot wrote:
... as well as the globular star cluster NGC 188 on the far left.

Nope. NGC 188 is an open cluster, and the Wikipedia page that the caption links to says so, too. According to Wikipedia, NGC 188 is older than the Sun:

Wikipedia wrote:
NGC 188 lies far above the plane of the galaxy and is one of the most ancient of open clusters known, at approximately 6.8 billion years old.

High school student Abigail Bohl and and her advisor Mark Brooks Hedstrom wrote:

NGC 188, one of the oldest open clusters in the Milky Way, provides a unique insight into the history of the universe.

NGC 188 has been historically used as a reliable test subject for various methods of determining data about star clusters. Because of its importance in the field of astronomical study and due to its unique age, this paper serves to independently verify the data found by previous research and to determine the age, metallicity, distance, and mass of open cluster NGC 188. Methods involved color magnitude diagram creation, isochrone fitting, and mass determination using the virial theorem. This research verifies that NGC 188 has an age of 6 billion years, a metallicity of 0.2 [Fe/H], a distance of 1348 parsecs, and a mass of 1392 solar masses.

file:///C:/Users/Ann%20Sidbrant/Downloads/2436-Article%20Text-14314-1-10-20220423%20(1).pdf

I like this picture of Polaris, NGC 188 and the Integrated Flux Nebula (the IFN), taken by 14-year-old Kush Chandaria:


Note how blue Polaris looks in Kush Chandaria's image. I have seen this effect in other pictures of Polaris too. Below is a brilliant picture of a blue-looking Polaris and Comet C/2014 Q2 Lovejoy passing within a degree of the bright star:


Awesome picture, isn't it? But why does Polaris look blue? It is not a blue star, as it is spectral class F8Ib, which makes it just a tiny bit bluer than the Sun. I guess it is all the IFN near Polaris that acts as a reflection nebula.


The picture of a blue-looking Polaris surrounded by all that IFN reminds me of star HD 200775 inside the Iris Nebula, surrounded by a lot of brown dust clouds:


Well, fascinating! There is a lot more to say particularly about Polaris. I must add that as usual, there is a mistake at Wikipedia. The luminosity of the brightest component of Polaris (which totally dominates the light output of this triple star system) is said to be 1,260 L. The source behind this figure is a 2005 paper. Well, the best available parallax for Polaris puts it at a distance of some 430 light-years, similar to the Pleiades, and coupled with its apparent luminosity, this translates to an absolute luminosity of some 2,000 L.

Okay! I throw in the towel now.

Ann
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Re: APOD: North Star: Polaris and Surrounding Dust (2023 Apr 11)

Post by johnnydeep » Tue Apr 11, 2023 7:58 pm

So, referencing Ann's dissertation above :ssmile:, is the age of a cluster (e.g., NGC 188) just the average (or perhaps most common) age of all the individual stars in it?
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Re: APOD: North Star: Polaris and Surrounding Dust (2023 Apr 11)

Post by Ann » Wed Apr 12, 2023 3:20 am

johnnydeep wrote: Tue Apr 11, 2023 7:58 pm So, referencing Ann's dissertation above :ssmile:, is the age of a cluster (e.g., NGC 188) just the average (or perhaps most common) age of all the individual stars in it?
That depends on whether NGC 188 is a one-generation or a multi-generation cluster. Most likely it's one-generation cluster, since most open clusters (and even most globular clusters) are.

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Re: APOD: North Star: Polaris and Surrounding Dust (2023 Apr 11)

Post by johnnydeep » Wed Apr 12, 2023 4:08 pm

Ann wrote: Wed Apr 12, 2023 3:20 am
johnnydeep wrote: Tue Apr 11, 2023 7:58 pm So, referencing Ann's dissertation above :ssmile:, is the age of a cluster (e.g., NGC 188) just the average (or perhaps most common) age of all the individual stars in it?
That depends on whether NGC 188 is a one-generation or a multi-generation cluster. Most likely it's one-generation cluster, since most open clusters (and even most globular clusters) are.

Ann
So then NGC 188 would refer to the average age of its constituent stars. What about the (globular) clusters - like NGC 6397 - that are similar in age to the Universe? ...Answering my own question with google:
https://astronomy.com/magazine/ask-astro/2019/02/age-of-a-globular-cluster wrote:A: To determine the age of a globular cluster, astronomers rely on the assumption that all the stars in the cluster formed at the same time and are the same distance from us. But the stars don’t all have the same mass. Because mass influences a star’s longevity, a star cluster gives us a special snapshot of a bunch of stars that are all the same age but at different points in their evolution.

Astronomers often use a plot called the Hertzprung-Russell (H-R) diagram. It measures the temperature (color) of a star versus its luminosity (intrinsic brightness).

More massive stars are hotter (bluer) and have higher luminosity. The smallest stars are cooler (redder) and have low luminosity.

Drawing a diagonal line across this diagram marks the main sequence. This is where stars spend the majority of their lives, fusing hydrogen in their cores. As the cluster ages, the massive, blue stars run out of fuel first. When a star dies, it stops fusing hydrogen and begins fusing helium. Because this happens at a much higher temperature, the star’s outer atmosphere expands, and it turns into a red giant. As the name suggests, a red giant is reddish in color, so a bright, blue star has now evolved into a bright, red star, and its place on the H-R diagram shifts.

While main sequence stars trace a diagonal line on the H-R diagram, red giant stars create a horizontal line in varying shades of red. This is called the red giant branch. As more of the big stars move onto the red giant branch, the top part of the main sequence disappears. This is called the main sequence turnoff. Because we know how a star’s mass determines how quickly it burns its fuel and where it falls on the diagram, we can use the main sequence turnoff to figure out the age of the cluster. For example, a bright O star will live about a million years. A G-type star like the Sun will live about 8 billion years. A faint, red M star can live 56 billion years. This means that even if it formed at the very start of the universe, an M star will still be on the main sequence today and give us good grounding for our H-R diagram.

NGC 6397 contains more than 400,000 stars, which gives us a lot of points. Using the color and magnitude of each star, we can convert to temperature and luminosity and put them on the H-R diagram. (Note: We technically need to know the distance in order to convert magnitude to luminosity, but since all the stars are the same distance, it doesn’t affect our age estimation.) By studying where the main sequence turnoff is and how far the stars that are no longer on the main sequence have evolved, scientists arrive at the cluster’s age of 13.4 billion years.
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}

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