APOD: Orion in Depth (2020 Sep 19)

[APOD Robot]

Orion in Depth

Explanation: Orion is a familiar constellation. The apparent positions of its stars in two dimensions create a well-known pattern on the bowl of planet Earth's night sky. Orion may not look quite so familiar in this 3D view though. The illustration reconstructs the relative positions of Orion's bright stars, including data from the Hipparcus catalog of parallax distances. The most distant star shown is Alnilam. The middle one in the projected line of three that make up Orion's belt when viewed from planet Earth, Alnilam is nearly 2,000 light-years away, almost 3 times as far as fellow belt stars Alnitak and Mintaka. Though Rigel and Betelgeuse apparently shine brighter in planet Earth's sky, that makes more distant Alnilam intrinsically (in absolute magnitude) the brightest of the familiar stars in Orion. In the Hipparcus catalog, errors in measured parallaxes for Orion's stars can translate in to distance errors of a 100 light-years or so.

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[BDanielMayfield]

Yesterday,

I think today's APOD has been replaced. Yesterday's APOD said that tomorrow's APOD (that is, today's APOD of September 18) would be Orion in 3D. I groaned, because I just can't deal with 3D pictures.

I kinda agreed, I don't much care for them either. But this isn't what we thought it might be, is it. Actually, I quite like this presentation of Orion. This is very informative.

Thanks for including this as an APOD.

[A vent]

Distant sky if such great past, the Ray's of such will never last. To us the familiar is as such. And much the day will hide.

[Donald Pelletier]

Distance on the illustration does not fit those of Hipparcos. The parallax of Alnilam on the catalog is 2,43 (under the entry 26311). This parallaxe correspond to a distance of (1/0.00243) pc = 411 pc or about 1350 light-year, not 1976 ly.

[Ann]

Yesterday,

I think today's APOD has been replaced. Yesterday's APOD said that tomorrow's APOD (that is, today's APOD of September 18) would be Orion in 3D. I groaned, because I just can't deal with 3D pictures.

I kinda agreed, I don't much care for them either. But this isn't what we thought it might be, is it. Actually, I quite like this presentation of Orion. This is very informative.

Thanks for including this as an APOD.

I really like it too! Except I don't believe in it!

Vela Supernova remnant with bright stars Gamma Velorum (bottom right) and Zeta Puppis (upper right). Image Credit & Copyright: Robert Gendler, Roberto Colombari, Digitized Sky Survey (POSS II).

---

Take a look at the picture at right of the Vela Supernova remnant. There are two bright blue stars there, Gamma Velorum at bottom right and Zeta Puppis at upper right. Gamma Velorum is noticeably brighter than Zeta Puppis.

These two stars are far away. When the Hipparcos satellite measured their parallaxes, their apparent "back and forth movement" across the sky as the Earth orbits the Sun, the measured parallaxes were so small that they were within the margin of error.

Anyway. The person analysing the Hipparcos data first thought that the parallax of Zeta Puppis was particularly small, and that Zeta Puppis was considerably farther away than Gamma Velorum. Because the truth is that a parallax of 1 milliarcsecond makes a huge difference in distance compared with a parallax of 2 milliarcseconds - but they are both completely within the margin of error.

Anyway, Zeta Puppis and Gamma Velorum. In 2008, Jim Kaler took the miniscule Hipparcos parallax of Zeta Puppis at face value and wrote the following:

Jim Kaler wrote about Zeta Puppis:

Its large distance of 1400 light years shows it to be visually 22,000 times more luminous than the Sun. Its high temperature of 42,000 Kelvin, however, causes most of the star's radiation to be emitted in the invisible ultraviolet, and when that is taken into account, the total luminosity comes in at three-quarters of a million times solar.

But the Hipparcos parallax of Zeta Puppis has since been revised. It is now deemed to just a little larger. And lo and behold, suddenly the distance to Zeta Puppis shrinks to 1084 ± 36 light-years (according to my Guide) and its visual luminosity drops by almost half, to 11990 ± 800 Solar.

The distance to Gamma Velorum hasn't been revised downwards, because its parallax hasn't been deemed to be larger than first assumed. So now Gamma Velorum is thought to be marginally farther away from us than Zeta Puppis.

Orion's Belt. Is the middle star really more than twice as far away from us as the two others? Photo: Bob King.

---

My point? Consider the parallaxes of the stars of Orion's Belt. The Hipparcos parallax of Alnitak is 4.43 ± 0.64 milliarcseconds. The Hipparcos parallax for Alnilam is 1.65 ± 0.45 mas, and the Hipparcos parallax for Mintaka is 4.71 ± 0.58 mas.

These parallaxes would put Alnilam, the middle star of Orion's Belt, more than twice as far away from us as the two other Belt stars.

Here's the deal. You can't trust those parallaxes. They are way too small to be reliable. Heck, Hipparcos isn't thought to have done a very good job at measuring the distance to the Pleiades, whose distance has long been thought to be around 440 light-years, much more nearby than Alnilam, Alnitak and Mintaka. Hipparcos put them at only about 400 light-years, or some 120 parsecs. The consensus is that the Hipparcos distance estimate for the Pleiades is wrong.

So today's APOD is fun, and I really appreciate it! But I must warn you: It shows you the distances of the stars of Orion as inferred from the parallaxes measured by Hipparcos, and they may very well be a far cry from reality.

So if Alnilam turns out to be at more or less the same distance from us as Alnitak and Mintaka, you shouldn't be surprised.

Ann

[BDanielMayfield]

Distance on the illustration does not fit those of Hipparcos. The parallax of Alnilam on the catalog is 2,43 (under the entry 26311). This parallaxe correspond to a distance of (1/0.00243) pc = 411 pc or about 1350 light-year, not 1976 ly.

Distance is what this APOD is all about, but there seems to be much uncertainty, particularly with the most distant object on this graph; the belt star Alnilam. So I googled Alnilam, and here is the definition google provided:

Alnilam, designation ε Orionis, and 46 Orionis, is a large blue supergiant star some 2,000 light-years distant in the constellation of Orion. It is estimated to be 275,000 to 832,000 times as luminous as the Sun, and 30–64.5 times as massive. Wikipedia
Distance to Earth: 1,344 light years
Luminosity: 832,000 L☉
Radius: 14.006 million mi
Surface temperature: 27,000 K
Distance: approx. 2,000 ly; (approx. 600 pc)
Spectral type: B0Ia

So is Alnilam about 1,344 light years away, or about 2,000 light years away?

[BDanielMayfield]

Consider the parallaxes of the stars of Orion's Belt. The Hipparcos parallax of Alnitak is 4.43 ± 0.64 milliarcseconds. The Hipparcos parallax for Alnilam is 1.65 ± 0.45 mas, and the Hipparcos parallax for Mintaka is 4.71 ± 0.58 mas.

These parallaxes would put Alnilam, the middle star of Orion's Belt, more than twice as far away from us as the two other Belt stars.

Here's the deal. You can't trust those parallaxes. They are way too small to be reliable. Heck, Hipparcos isn't thought to have done a very good job at measuring the distance to the Pleiades, whose distance has long been thought to be around 440 light-years, much more nearby than Alnilam, Alnitak and Mintaka. Hipparcos put them at only about 400 light-years, or some 120 parsecs. The consensus is that the Hipparcos distance estimate for the Pleiades is wrong.

So today's APOD is fun, and I really appreciate it! But I must warn you: It shows you the distances of the stars of Orion as inferred from the parallaxes measured by Hipparcos, and they may very well be a far cry from reality.

So if Alnilam turns out to be at more or less the same distance from us as Alnitak and Mintaka, you shouldn't be surprised.

Ann

You've well detailed the problem here Ann. It is the uncertainties in the Hipparcos data. The distance estimates used in this graphic are based on a 2007 revision of the Hipparcos data, which, unfortunately, is still the data that is most widely available.

Where's the GAIA results?! We need better data.

[orin stepanek]

Wouldn't it be interesting To see Orion's stars in 360 degree rotations?

[Phobos1]

This presentation of Orion would make a beautiful large scale artwork. With properly intense and colored LEDs, you could fill a room with it, and there would be one location from which you could see familiar old Orion. -Given the distances, that spot might have to be down a hallway.

[Chris Peterson]

Consider the parallaxes of the stars of Orion's Belt. The Hipparcos parallax of Alnitak is 4.43 ± 0.64 milliarcseconds. The Hipparcos parallax for Alnilam is 1.65 ± 0.45 mas, and the Hipparcos parallax for Mintaka is 4.71 ± 0.58 mas.

These parallaxes would put Alnilam, the middle star of Orion's Belt, more than twice as far away from us as the two other Belt stars.

Here's the deal. You can't trust those parallaxes. They are way too small to be reliable. Heck, Hipparcos isn't thought to have done a very good job at measuring the distance to the Pleiades, whose distance has long been thought to be around 440 light-years, much more nearby than Alnilam, Alnitak and Mintaka. Hipparcos put them at only about 400 light-years, or some 120 parsecs. The consensus is that the Hipparcos distance estimate for the Pleiades is wrong.

So today's APOD is fun, and I really appreciate it! But I must warn you: It shows you the distances of the stars of Orion as inferred from the parallaxes measured by Hipparcos, and they may very well be a far cry from reality.

So if Alnilam turns out to be at more or less the same distance from us as Alnitak and Mintaka, you shouldn't be surprised.

Ann

You've well detailed the problem here Ann. It is the uncertainties in the Hipparcos data. The distance estimates used in this graphic are based on a 2007 revision of the Hipparcos data, which, unfortunately, is still the data that is most widely available.

Where's the GAIA results?! We need better data.

To a large extent, though, I don't think it matters for the purposes of this image, which is really just about demonstrating the way that a familiar constellation, which we perceive as lying on a plane, is really a projection of unrelated stars at very different distances. That message is the same regardless of the strict accuracy of the distances.

[jisles]

Note that it's the Hipparcos Catalogue - not Hipparcus as given twice in the caption to today's APOD.

John

[neufer]

The distance estimates used in this graphic are based on a 2007 revision of the Hipparcos data, which, unfortunately, is still the data that is most widely available.

Where's the GAIA results?! We need better data.

<<Although it was originally planned to limit Gaia's observations to stars fainter than magnitude 5.7, tests carried out during the commissioning phase indicated that Gaia could autonomously identify stars as bright as magnitude 3.

When Gaia entered regular scientific operations in July 2014, it was configured to routinely process stars in the magnitude range 3 – 20. Beyond that limit, special procedures are used to download raw scanning data for the remaining 230 stars brighter than magnitude 3; methods to reduce and analyse these data are being developed; and it is expected that there will be "complete sky coverage at the bright end" with standard errors of "a few dozen µas".>>

[E Fish]

I wish I'd had this APOD at the beginning of the week when one of my students was asking about why there are constellations. I used an illustration of the Big Dipper that I found through a quick Google Search, but I really like this. Even if the distances aren't quite right, the demonstration that it's just the stars that happen to be in the same direction is really effective here. I like having the whole constellation and then, the angle shot to reveal that these stars are, in some case, nowhere near each other.

Well, when we get to astronomy again at the end of the semester, maybe I'll use this one to talk about it more.

[Chris Peterson]

Wouldn't it be interesting To see Orion's stars in 360 degree rotations? 8-)

Click to play embedded YouTube video.

[orin stepanek]

Wouldn't it be interesting To see Orion's stars in 360 degree rotations?

Click to play embedded YouTube video.

Wow! Thanks Chris; that's just wonderful! How things look out there's a whole new ballgame !

[BDanielMayfield]

Consider the parallaxes of the stars of Orion's Belt. The Hipparcos parallax of Alnitak is 4.43 ± 0.64 milliarcseconds. The Hipparcos parallax for Alnilam is 1.65 ± 0.45 mas, and the Hipparcos parallax for Mintaka is 4.71 ± 0.58 mas.

These parallaxes would put Alnilam, the middle star of Orion's Belt, more than twice as far away from us as the two other Belt stars.

Here's the deal. You can't trust those parallaxes. They are way too small to be reliable. Heck, Hipparcos isn't thought to have done a very good job at measuring the distance to the Pleiades, whose distance has long been thought to be around 440 light-years, much more nearby than Alnilam, Alnitak and Mintaka. Hipparcos put them at only about 400 light-years, or some 120 parsecs. The consensus is that the Hipparcos distance estimate for the Pleiades is wrong.

So today's APOD is fun, and I really appreciate it! But I must warn you: It shows you the distances of the stars of Orion as inferred from the parallaxes measured by Hipparcos, and they may very well be a far cry from reality.

So if Alnilam turns out to be at more or less the same distance from us as Alnitak and Mintaka, you shouldn't be surprised.

Ann

You've well detailed the problem here Ann. It is the uncertainties in the Hipparcos data. The distance estimates used in this graphic are based on a 2007 revision of the Hipparcos data, which, unfortunately, is still the data that is most widely available.

Where's the GAIA results?! We need better data.

To a large extent, though, I don't think it matters for the purposes of this image, which is really just about demonstrating the way that a familiar constellation, which we perceive as lying on a plane, is really a projection of unrelated stars at very different distances. That message is the same regardless of the strict accuracy of the distances.

I agree, "for the purposes of this image", so this is a good selection as an APOD. It helps people learn to think more in 3D when looking at Orion and other stellar groupings.

[BDanielMayfield]

Wouldn't it be interesting To see Orion's stars in 360 degree rotations?

Click to play embedded YouTube video.

Wow! Thanks Chris; that's just wonderful! How things look out there's a whole new ballgame !

Yes that is a very nice little vid. Thanks for sharing it Chris.

[heehaw]

I think the main great thing about Orion is that it is a bit of the galaxy that is tipped OUT of the plane, so we can see it without a lot of stuff between us and it. That makes it much easier to study and analyze.

[RC Davison]

Thanks to all for the engaging discussion!

The image's main purpose is to enlighten people that there is depth in the cosmos and the stars of the constellations are not at a fixed distance. Hopefully it has achieved that goal.

I went to Gaia's data first thinking that it would be the most accurate, but alas, the stars of Orion were too bright to be included in the released data sets. It's interesting to see that that data may be coming, and maybe we'll get more precise information on Orion's stars. Looking at other sources for distances I found a lack of agreement between sites. Alnilam, 1350 or 1976 ly??? It was driving me crazy! So I decided on using the 2007 reduction of the Hipparcos data for the image and to make sure it was referenced on the image to help people understand where the distances came from.

There is a lot of potential for error to creep into these small parallax measurements and this has dramatic effects in the distance and brightness of the stars. In the end it's the data we have to use and we can expect it to change as our technology improves.

Thanks,
RC Davison