APOD: Zeta Oph: Runaway Star (2020 Feb 02)

[APOD Robot]

Zeta Oph: Runaway Star

Explanation: Like a ship plowing through cosmic seas, runaway star Zeta Ophiuchi produces the arcing interstellar bow wave or bow shock seen in this stunning infrared portrait. In the false-color view, bluish Zeta Oph, a star about 20 times more massive than the Sun, lies near the center of the frame, moving toward the left at 24 kilometers per second. Its strong stellar wind precedes it, compressing and heating the dusty interstellar material and shaping the curved shock front. What set this star in motion? Zeta Oph was likely once a member of a binary star system, its companion star was more massive and hence shorter lived. When the companion exploded as a supernova catastrophically losing mass, Zeta Oph was flung out of the system. About 460 light-years away, Zeta Oph is 65,000 times more luminous than the Sun and would be one of the brighter stars in the sky if it weren't surrounded by obscuring dust. The image spans about 1.5 degrees or 12 light-years at the estimated distance of Zeta Ophiuchi. Last week, NASA placed the Spitzer Space Telescope in safe mode, ending its 16 successful years of studying our universe.

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[Alex515]

Hi,

The image is both gorgeous and impressive when seen at full resolution. What are the two red dots seen right and left of the arc bow ?There are also some green dots in the background, they look more like background galaxies.

Alex

[Ann]

Hi,

The image is both gorgeous and impressive when seen at full resolution. What are the two red dots seen right and left of the arc bow ?There are also some green dots in the background, they look more like background galaxies.

Alex

Hi Alex! You gave me a tough nut to crack there. In fact, I wasn't able to crack it!

As for the two red dots, they are most likely some highly dust-reddened objects that were detected by Spitzer's longest wavelength filter. To me they don't look like baby stars in the process of forming, though.

I agree with you that the green objects at 8 o'clock (or 8.30) and 6 o'clock (or 6.30) look very much like galaxies. I'd be very happy to tell you the designations of these galaxies, if I knew where to look for them so that I could find them and identify them. I find it very hard to "translate" this Spitzer image into a visual image that I can use to estimate the positions of the galaxies, so I would know where to look for them.

Click to view full size image

Zeta Ophiuchi in infrared light.
Photo: NASA, JPL-Caltech, Spitzer Space Telescope

Click to view full size image

From Rho to Zeta Ophiuchi and the Galactic center.
Photo: Scott Rosen. Full size here.

If you take a look at Scott Rosen's image at right, you can easily find Zeta Ophiuchi. It is the blue-white star at right that is surrounded by a very big red emission nebula. If you look carefully, you should be able to see an rather long undulating dust lane cutting through the upper part of this red nebula, relatively far from Zeta Ophiuchi. Is this undulating dust lane the glowing shock front that we can see in today's APOD? Who knows?

You can see that there is a "fainter but still bright" blue star to the left of the glowing wall in the APOD. Is that the blue-white star that you can see in Scott Rosen's image quite far to the upper left of Zeta Ophiuchi, well away from the large red nebula? Who knows? But if "the other blue star" in today's APOD really is that blue-white star in Scott Rosen's image, then the star is Eta Ophiuchi.

That's as far as I can get, sorry. I haven't been able to find any background galaxies in the vicinity of Eta Ophiuchi. You should bear in mind, too, that at the position of Eta we are getting closer and closer to the center of the Milky Way, and we can expect all sorts of interesting objects to be located there in our own galaxy.

Ann

[orin stepanek]

So does Zeta Oph eventually find it's place in the galaxy and find a normal orbit?

[Chris Peterson]

So does Zeta Oph eventually find it's place in the galaxy and find a normal orbit? :shock:

This star is already in a "normal orbit". Its orbital speed is around 230 km/s, and is about 24 km/s different from the surrounding medium. Indeed, that's not so different from the Sun, which is moving 20 km/s faster than our surrounding medium. "Runaway" sounds impressive, but it's just a small difference in speed from normal, and wouldn't really be noticed except for a rich enough medium around it to show shock fronts.

[rstevenson]

Hi,

The image is both gorgeous and impressive when seen at full resolution. What are the two red dots seen right and left of the arc bow ?There are also some green dots in the background, they look more like background galaxies.

Alex

Hi Alex! You gave me a tough nut to crack there. In fact, I wasn't able to crack it!

As for the two red dots, they are most likely some highly dust-reddened objects that were detected by Spitzer's longest wavelength filter. To me they don't look like baby stars in the process of forming, though.

I agree with you that the green objects at 8 o'clock (or 8.30) and 6 o'clock (or 6.30) look very much like galaxies. I'd be very happy to tell you the designations of these galaxies, if I knew where to look for them so that I could find them and identify them. I find it very hard to "translate" this Spitzer image into a visual image that I can use to estimate the positions of the galaxies, so I would know where to look for them.

...

Ann

I had a look at these objects in my Digital Color Meter, which uses a very magnified view to allow precise positioning of its crosshairs. The green objects appear just like Zeta Oph: equal amounts of blue and green, with zero red. So I'm guessing they (at least the two brighest ones, one to the left and one top-right) are just stars. The two red dots, on the other hand could conceivably be background (very!) galaxies, but I think not, not in that part of the sky. So if they're not imaging artifacts, they must be small dark stars. The elongated green blob (also equally green and blue with no red) near the bottom center of the image is a galaxy, judging from its cigar shape.

The most interesting thing I noticed is that all the objects I looked at (except the cigar) show what appears to be a subtle three-lobed shape, and all of them have the lobes oriented the same way. So I'm guessing that there are three almost perfectly aligned images used to show at least the background stars.

Rob

[bystander]

viewtopic.php?t=40205

[neufer]

Click to play embedded YouTube video.

So does Zeta Oph eventually find it's place in the galaxy and find a normal orbit?

This star is already in a "normal orbit". Its orbital speed is around 230 km/s, and is about 24 km/s different from the surrounding medium. Indeed, that's not so different from the Sun, which is moving 20 km/s faster than our surrounding medium. "Runaway" sounds impressive, but it's just a small difference in speed from normal, and wouldn't really be noticed except for a rich enough medium around it to show shock fronts.

.
Our Sun has a stable orbital velocity of 220 km/s
27,200 ly from the center of a "quasi-logarithmic" galactic gravitational
potential well that extends roughly out to 75,000 ly.

A back of the envelope calculation of the galactic escape velocity from the vicinity of the Sun is:

• galactic escape velocity ~ sqrt[1+2ln(75,000 ly/27,200 ly)] x 220 km/s
  galactic escape velocity ~ sqrt[3] x 220 km/s = 380 km/s

This is roughly consistent with the velocity of the Magellanic Stream:

Map of Hubble observations overlaid on LAB survey, tracing the origin of the Magellanic Stream.

---

<<The Magellanic Stream, which contains a gaseous feature dubbed the leading arm, is a stream of high-velocity clouds of gas extending from the Large and Small Magellanic Clouds over 100° through the Galactic south pole of the Milky Way. The stream was sighted in 1965 and its relation to the Magellanic Clouds was established in 1974.

In 1965, anomalous velocity gas clouds were found in the region of the Magellanic Clouds. The gas stretches for at least 180 degrees across the sky. This corresponds to 600,000 ly at an approximate distance of 180,000 ly. The gas is very collimated and polar with respect to the Milky Way. The velocity range is huge (from −400 to 400 km/s in reference to Local Standard of Rest) and velocity patterns do not follow the rest of the Milky Way. Hence, it was determined to be a classic high-velocity cloud.

However, the gas was not mapped, and the connection to the two Magellanic Clouds was not made. The Magellanic Stream as such was discovered as a Neutral Hydrogen (HI) gas feature near the Magellanic Clouds by Wannier & Wrixon in 1972. Its connection to the Magellanic Clouds was made by Mathewson et al. in 1974.

Owing to the closeness of the Magellanic Clouds and the ability to resolve individual stars and their parallaxes, and proper motion, subsequent observations gave the full 6-dimensional phase space information of both clouds (with very large relative errors for the transverse velocities). This enabled the calculation of the likely past orbit of the Large and the Small Magellanic Cloud in relation to the Milky Way. The calculation necessitated large assumptions, for example, on the shapes and masses of the 3 galaxies, and the nature of dynamical friction between the moving objects. Observations of individual stars revealed details of star formation history.>>

[Chris Peterson]

A back of the envelope calculation of the galactic escape velocity from the vicinity of the Sun is:

• galactic escape velocity ~ sqrt[1+2ln(75,000 ly/27,200 ly)] x 220 km/s
  galactic escape velocity ~ sqrt[3] x 220 km/s = 380 km/s

The actual value for an object at the radius of the Sun in galactic orbit is 500-600 km/s. The uncertainty (and deviation from what might be expected) is a consequence of the large mass outside our orbit, and is part of the strong evidence for a dark matter halo around the galaxy.

[neufer]

A back of the envelope calculation of the galactic escape velocity from the vicinity of the Sun is:

• galactic escape velocity ~ sqrt[1+2ln(75,000 ly/27,200 ly)] x 220 km/s
  galactic escape velocity ~ sqrt[3] x 220 km/s = 380 km/s

The actual value for an object at the radius of the Sun in galactic orbit is 500-600 km/s. The uncertainty (and deviation from what might be expected) is a consequence of the large mass outside our orbit, and is part of the strong evidence for a dark matter halo around the galaxy.

I did say that it was a back of the envelope calculation of the galactic escape velocity.

However, I should really have written:

• galactic escape velocity ~ sqrt[2+2ln(75,000 ly/27,200 ly)] x 220 km/s
  galactic escape velocity ~ sqrt[4] x 220 km/s = 440 km/s

[Chris Peterson]

A back of the envelope calculation of the galactic escape velocity from the vicinity of the Sun is:

• galactic escape velocity ~ sqrt[1+2ln(75,000 ly/27,200 ly)] x 220 km/s
  galactic escape velocity ~ sqrt[3] x 220 km/s = 380 km/s

The actual value for an object at the radius of the Sun in galactic orbit is 500-600 km/s. The uncertainty (and deviation from what might be expected) is a consequence of the large mass outside our orbit, and is part of the strong evidence for a dark matter halo around the galaxy.

I did say that it was a back of the envelope calculation of the galactic escape velocity.

However, I should really have written:

• galactic escape velocity ~ sqrt[2+2ln(75,000 ly/27,200 ly)] x 220 km/s
  galactic escape velocity ~ sqrt[4] x 220 km/s = 440 km/s

To be sure, I wasn't challenging the calculation, which was reasonable. Just thought it was interesting that people have worked this out using numerical simulations as well as observations of a variety of actual high speed objects, and interesting the impact of the dark matter halo.

[neufer]

The actual value for an object at the radius of the Sun in galactic orbit is 500-600 km/s. The uncertainty (and deviation from what might be expected) is a consequence of the large mass outside our orbit, and is part of the strong evidence for a dark matter halo around the galaxy.

I did say that it was a back of the envelope calculation of the galactic escape velocity.

However, I should really have written:

• galactic escape velocity ~ sqrt[2+2ln(75,000 ly/27,200 ly)] x 220 km/s
  galactic escape velocity ~ sqrt[4] x 220 km/s = 440 km/s

To be sure, I wasn't challenging the calculation, which was reasonable. Just thought it was interesting that people have worked this out using numerical simulations as well as observations of a variety of actual high speed objects, and interesting the impact of the dark matter halo.

Indeed...it would have been better to have used the latest
estimates of total Milky Way mass = 800-1500 x 10⁹ M_☉ compared
against the mere ~91 x 10⁹ M_☉ of galactic mass interior to the Sun (given its orbital velocity)

...in which case my "quasi-logarithmic" potential well calculations would yield:

• galactic escape velocity ~ sqrt[2+2ln(800/91)] x 220 km/s
  galactic escape velocity ~ 555 km/s

• galactic escape velocity ~ sqrt[2+2ln(1500/91)] x 220 km/s
  galactic escape velocity ~ 607 km/s

A more realistic potential well
(i.e., one that allowed the "constant" 220 km/s orbital velocity to drop off gradually)
would yield the somewhat lower velocities you cite.

[orin stepanek]

So does Zeta Oph eventually find it's place in the galaxy and find a normal orbit?

This star is already in a "normal orbit". Its orbital speed is around 230 km/s, and is about 24 km/s different from the surrounding medium. Indeed, that's not so different from the Sun, which is moving 20 km/s faster than our surrounding medium. "Runaway" sounds impressive, but it's just a small difference in speed from normal, and wouldn't really be noticed except for a rich enough medium around it to show shock fronts.

Thanks Chris!

[Alex515]

Hi,

The image is both gorgeous and impressive when seen at full resolution. What are the two red dots seen right and left of the arc bow ?There are also some green dots in the background, they look more like background galaxies.

Alex

Hi Alex! You gave me a tough nut to crack there. In fact, I wasn't able to crack it!

As for the two red dots, they are most likely some highly dust-reddened objects that were detected by Spitzer's longest wavelength filter. To me they don't look like baby stars in the process of forming, though.

I agree with you that the green objects at 8 o'clock (or 8.30) and 6 o'clock (or 6.30) look very much like galaxies. I'd be very happy to tell you the designations of these galaxies, if I knew where to look for them so that I could find them and identify them. I find it very hard to "translate" this Spitzer image into a visual image that I can use to estimate the positions of the galaxies, so I would know where to look for them.

Click to view full size image

Zeta Ophiuchi in infrared light.
Photo: NASA, JPL-Caltech, Spitzer Space Telescope

Click to view full size image

From Rho to Zeta Ophiuchi and the Galactic center.
Photo: Scott Rosen. Full size here.

If you take a look at Scott Rosen's image at right, you can easily find Zeta Ophiuchi. It is the blue-white star at right that is surrounded by a very big red emission nebula. If you look carefully, you should be able to see an rather long undulating dust lane cutting through the upper part of this red nebula, relatively far from Zeta Ophiuchi. Is this undulating dust lane the glowing shock front that we can see in today's APOD? Who knows?

You can see that there is a "fainter but still bright" blue star to the left of the glowing wall in the APOD. Is that the blue-white star that you can see in Scott Rosen's image quite far to the upper left of Zeta Ophiuchi, well away from the large red nebula? Who knows? But if "the other blue star" in today's APOD really is that blue-white star in Scott Rosen's image, then the star is Eta Ophiuchi.



That's as far as I can get, sorry. I haven't been able to find any background galaxies in the vicinity of Eta Ophiuchi. You should bear in mind, too, that at the position of Eta we are getting closer and closer to the center of the Milky Way, and we can expect all sorts of interesting objects to be located there in our own galaxy.

Ann

Thanks so much for the detailed answer Ann !

Alex