APOD: Epsilon Tauri: Star with Planet (2024 Jan 26)

[APOD Robot]

Epsilon Tauri: Star with Planet

Explanation: Epsilon Tauri lies 146 light-years away. A K-type red giant star, epsilon Tau is cooler than the Sun, but with about 13 times the solar radius it has nearly 100 times the solar luminosity. A member of the Hyades open star cluster the giant star is known by the proper name Ain, and along with brighter giant star Aldebaran, forms the eyes of Taurus the Bull. Surrounded by dusty, dark clouds in Taurus, epsilon Tau is also known to have a planet. Discovered by radial velocity measurements in 2006, Epsilon Tauri b is a gas giant planet larger than Jupiter with an orbital period of 1.6 years. And though the exoplanet can't be seen directly, on a dark night its parent star epsilon Tauri is easily visible to the unaided eye.

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[Ann]

APOD Robot wrote:

Epsilon Tauri lies 146 light-years away. A K-type red giant star, epsilon Tau is cooler than the Sun, but with about 13 times the solar radius it has nearly 100 times the solar luminosity.

Color Commentator asks: This is a red giant star?

Ann

[Knight of Clear Skies]

Color Commentator asks: This is a red giant star?

Ann

I wondered if the image had been mislabelled but looking on DSS the starfield matches. It's a nice image with the star framed by the dust but looks like something has gone badly wrong with the colour calibration. Which is strange, as there are plenty of orange stars in the background.

False and enhanced colour serve a purpose, allowing us to see features that would be invisible or indistinct. I'm not sure what's going on with this image and am not convinced it's consistent within the frame. I'm wondering if the bright star has been processed as a separate layer from the background.

[Rauf]

https://www.r3gdigital.com/neb-d-details-01/ain

Teleskop-Service ONTC 254mm
Teleskop-Service GPU Coma Corrector
IMX571, Antlia V-Pro LRGB
iOptron CEM70

L: 22.87
R: 5.95
G: 5.33
B: 5.59

Total: 39.74 hrs

This is the equipment that the astrophotographer used, according to his website. Does these possibly affect the star colour?

[RegP]

The color isn't false, I ran the RGB through SPCC in Pixinsight. I was surprised at the result as well. My understanding is that the surrounding dust can effect the appearance of the starlight.

[Chris Peterson]

The color isn't false, I ran the RGB through SPCC in Pixinsight. I was surprised at the result as well. My understanding is that the surrounding dust can effect the appearance of the starlight.

How long were your subs (especially the lum subs), and are you sure none of them resulted in the star being saturated?

[RegP]

Subs were 30s long with around 200 saturated pixels. Also, if you look up other broadband images from this area you'll find many of them also show Ain as blue.

I don't have a scientific explanation as to why it's blue even after color calibration all I know is I didn't mislead, falsify, or enhance the color.

[MrProulx]

It's obvious some people here don't know much about long exposure, spectroscopy and post processing. This star is not overexposed it's just stretched, mainly because the imager wanted to showcase the surrounding dust. If you actually look at the data raw or lower the exposure you'll see an orange star. I do spectroscopy and a star of this class and brightness only needs a few seconds of exposure to show it's proper color but for astrophotography purposes it would not be enough. The result will be different no matter what routine you run on it. The surrounding dust and the background will affect the color of the flare which by no means is required for accuracy in spectroscopy because you would not expose for this long.

[Chris Peterson]

Subs were 30s long with around 200 saturated pixels. Also, if you look up other broadband images from this area you'll find many of them also show Ain as blue.

I don't have a scientific explanation as to why it's blue even after color calibration all I know is I didn't mislead, falsify, or enhance the color.

I certainly have no reason to think you did anything to mislead, deliberately or otherwise. I've seen images of this star that cover a wide range of colors. I also use SPCC with my images, and have observed it to give very good results. Obviously saturated pixels won't color calibrate properly. Usually with bright stars we then see accurate color in the halo around the saturated core, which in your image is clearly blue. It's a dusty region, but usually starlight through dust reddens it, so blue is unexpected.

That said, this is not an orange star. It is at most a warmly tinted white (the same as Pollux, which few people see as anything other than neutral). And I might see hints of that in some of the artifact structure (mostly scatter, I'd guess) around the star. So I'm not convinced the star color is wrong here. It's very unsaturated because it is bright, and we may be fooled by the surrounding blue, which could be scattered from thin dust (and therefore bluish) rather than reflected from thicker dust (which would result in red), and therefore not representative of the star color itself.

[Ann]

Subs were 30s long with around 200 saturated pixels. Also, if you look up other broadband images from this area you'll find many of them also show Ain as blue.

I don't have a scientific explanation as to why it's blue even after color calibration all I know is I didn't mislead, falsify, or enhance the color.

The actual color of Ain is a little yellower than Pollux. It is not deeply orange, but it definitely isn't blue.

I agree with the photographer than the dust around Ain could in fact possibly create a blue cast in some instances.

Consider the Pole star, Polaris. It's a star of spectral class F8. whose color is quite similar to the Sun. It should photograph as white with perhaps just a hint of yellow, like this:

Polaris. Credit: NASA/ESA/HST, G. Bacon (STScI)

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But there are a lot of pictures where Polaris looks blue, such as this one:

Polaris.

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A very interesting portrait of a blue-looking Polaris was Astronomy Picture of the Day on June 2, 2015. Photographer Rogelio Bernal Andreo brought out a lot of galactic cirrus seen in (more or less) the same part of the sky as Polaris. Even though I don't think that the bluish halo surrounding Polaris in Bernal Andreo's image really looks like a reflection nebula, I still find it intriguing to consider the idea that dust reflection may play a part in creating a (faint) blue hue.

Polaris. Credit: Rogelio Bernal Andreo.

Just like Polaris, Ain really is surrounded by nebulosity:

Ain is the star immediately above bright orange Aldebaran in this image. Note the dust clouds seen to the right of Ain. Credit: Amir H. Abolfath (TWAN)

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However, Ain is definitely a redder star than Polaris, and it will be a lot less likely to create a blue reflection nebula. And while the photographer claims that that there are several pictures of Ain where the star looks blue, I myself have found exactly one such picture:

Ain is the topmost star in this image.

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As a Color Commentator, I look carefully at the colors of stars, clusters, nebulas and galaxies in astroimages. I am well aware that there are images where the colors of deep sky objects look strange indeed. What do you think of this portrait of the magenta Pleiades?

Magenta Pleiades. Credit: I don't know.

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And as for Ain, epsilon Tauri, the best color portrait I have found of it is this one:

Ain, epsilon Tauri. Credit: in-the-sky.org

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I think that the above picture makes Ain look a little bit too orange.

Ann

[Chris Peterson]

Ain, epsilon Tauri. Credit: in-the-sky.org

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I think that the above picture makes Ain look a little bit too orange.

Not a "bit too orange". Way, way too orange. This is a warm white star. Visually identical to Pollux... and you don't see people describe that as orange.

[RegP]

Here's an image from Adam Block of the same area and in his photo Ain also has a blue cast to it.

https://www.astrobin.com/b6pkft/


Again I'm not a scientist nor do I do spectroscopy. I thought it was curious as well given Ain is a K0III star but between SPCC, Adam, and other results I came across during this project I felt confident leaving it as is and chalking it up to the interaction between the starlight and surrounding dust.

My main point here was just to clarify that I did indeed color calibrate the image and no falsifying or enhancing of the color took place as was suggested above.

[Chris Peterson]

Here's an image from Adam Block of the same area and in his photo Ain also has a blue cast to it.

https://www.astrobin.com/b6pkft/


Again I'm not a scientist nor do I do spectroscopy. I thought it was curious as well given Ain is a K0III star but between SPCC, Adam, and other results I came across during this project I felt confident leaving it as is and chalking it up to the interaction between the starlight and surrounding dust.

My main point here was just to clarify that I did indeed color calibrate the image and no falsifying or enhancing of the color took place as was suggested above.

Keep in mind that this star sits on the G/K boundary. It is also classified as spectral class G9. Which is essentially the same as K0.

[RegP]

That makes sense as does Ann pointing out that dust can have an effect on the color of a stars halo. Actually I didn't even question it during the data collection and preliminary edits. I just ran SPCC on the linear data and trusted the result. It wasn't until I started collecting facts for my social media post (different from the APOD description) that I learned that the star was actually white/pale yellow. I didn't have a good answer to what was going on so I just trusted SPCC and ran with it.

[Javachip3]

Just to clarify, the prominent point of light at 3 o'clock to Epsilon Tauri, is not the planet, correct?

[Knight of Clear Skies]

Just to clarify, the prominent point of light at 3 o'clock to Epsilon Tauri, is not the planet, correct?

That's correct, very few exoplanets have been directly imaged and I don't believe any are within reach of an amateur telescope. The planet was detected in 2006 using the radial velocity method.

https://exoplanets.nasa.gov/alien-world ... -a-planet/

[Knight of Clear Skies]

That makes sense as does Ann pointing out that dust can have an effect on the color of a stars halo.

Thanks for clarifying. Dust is a possibility but I also wonder if it could be an optical artifact, perhaps a reflection in a filter. It's very challenging to take such a deep image with such a bright star in frame.

I also see Ani is classified as an orange rather than red giant, so the discrepancy in colour is not as much as I first thought. It has a surface temperature of 5,000K, so about 1,000K lower than the Sun, compared to 3,200-2,200K for a red giant.

[Ann]

That makes sense as does Ann pointing out that dust can have an effect on the color of a stars halo. Actually I didn't even question it during the data collection and preliminary edits. I just ran SPCC on the linear data and trusted the result. It wasn't until I started collecting facts for my social media post (different from the APOD description) that I learned that the star was actually white/pale yellow. I didn't have a good answer to what was going on so I just trusted SPCC and ran with it.

A dusty reflection nebula will redden the star providing the light for the nebula, but the light that is scattered by the dust is bluer than the unreddened star. In other words, shortwave light from the star is being scattered around the star by dust grains.

A prime example of the reddening of a star along with the "blueing" of its nebula is rho Ophiuchi and its nebula, IC 4604:

Star rho Ophiuchi and its nebula IC 4604. The star has been very considerably reddened, but the nebula is large and blue. Credit: Capturing ancient photons.

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The star rho Ophiuchi is spectral class B2, which means that its temperature is around 20,000 K, but dust has reddened it to a color that is typical of a star of around spectral class A7 and a temperature of around 8000 K. The "missing" blue light of rho Ophiuchi has been spread out around the star in a very large blue reflection nebula.

As for Ain, my software informs me that there is a nebula surrounding this star, Lynd's Bright Nebula 819. I also find it interesting that Ain is the (bright) member of the Hyades that has the reddest colors (apart from, obviously, nonmember Aldebaran). So I don't find it unreasonable to think that a bit of Ain's blue light has indeed been spread out around this star by dust. Because, yes, at a temperature of some 4,900 K Ain is hot enough and "white enough" to produce a non-negligible amount of blue light. In other words, there really might a (very faint) blue nebula surrounding Ain, and a very deep image of this star may bring out the nebula.

Blackbody curves of stars of different temperatures.

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Ann

[Ann]

Just to clarify, the prominent point of light at 3 o'clock to Epsilon Tauri, is not the planet, correct?

No, that's another star, HD 28139. According to the Hipparcos parallax of Ain, and the Gaia parallax of HD 28139, the small star is some 50 light-years farther away from us than Ain. In other words, HD 28138 would seem to be a background star. Its proper motion is also different than Ain's.

As for Ain's planet, no amateur equipment could bring it out. Remember that the planet is dark, or faintly lit up by Ain, whereas the star itself is (obviously) blindingly bright. Also the planet must be very very close to the star.

Seeing the planet next to Ain is many, many, many times harder than seeing a tiny candle a millimeter away from brilliant floodlights.

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