Very high motion star?

So a typical velocity of a star going around the galaxy is about 20km/sec. I believe that’s how fast our star goes around the merry go round.

IAE, I was perusing an old photo of Orion from 1908 and was wondering if there’ be been any changes in the clouds or stars. I found a 2016 picture of Orion that was nice. The only change I found is a star that’s shifted as can be seen circled below. The one on the left seems to go down by about 30 arcseconds over 108 years.

The star appears to be V1044 Orionis, an eruptive variable that is 1281 light years away.

But at that distance and time, that corresponds to over 500km/s!!! WTF? Even half that distance seems waaaaay too high. Has this ever been shown?

How sure are you of the identification?

Shadowy Man said:

How sure are you of the identification? What’s the HD number?

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Pretty sure. Here’s a screenshot from SkySafari.



But no HD number. There’s a TYC number.

Yeah. I couldn’t find an HD number. I confirmed your calculation of velocity given 30 arcseconds in 108 years but I haven’t found any reference giving a proper motion yet.

Probably a dumb question.

What is the reference point for astronomical motion and velocity?

From BB objects are moving away from each in relative velocity.

steve_bank said:

Probably a dumb question.

What is the reference point for astronomical motion and velocity?

From BB objects are moving away from each in relative velocity.

Click to expand...

"Proper motion" is the change in viewing angle from Earth, so it is effectively motion on the two dimensional surface of a hypothetical sphere, centered on the observer.

The reference against which proper motion is measured is what used to be called "fixed stars": Those objects that are so distant that they do not show any measurable "proper motion" relative to each other. Quasars and distant galaxies are so distant that their proper motions relative to each other are effectively zero.

The tangential motion of a star can be calculated if one knows the proper motion and the distance; Using red (or blue) shift data we can get the star's rate of motion away from (or towards) the observer, and combining these two measures allows us to calculate the direction and speed of motion in three dimensions of a star, relative to the Sun.

Proper motion should be measured using observations taken at the same time of year, to eliminate apparent motion due to parallax (or calculated using observations over a long period of time, so that parallax can be measured and subtracted), though this is less important the more distant the object.

steve_bank said:

Probably a dumb question.

What is the reference point for astronomical motion and velocity?

From BB objects are moving away from each in relative velocity.

Click to expand...

Here we are talking about within the galaxy, so the Hubble flow is not relevant. We can consider a heliocentric reference point or what is called the “local standard of rest”.

Shadowy Man said:

steve_bank said:

Probably a dumb question.

What is the reference point for astronomical motion and velocity?

From BB objects are moving away from each in relative velocity.

Click to expand...

Here we are talking about within the galaxy, so the Hubble flow is not relevant. We can consider a heliocentric reference point or what is called the “local standard of rest”.

Click to expand...

Got it thanks.

SLD said:

So a typical velocity of a star going around the galaxy is about 20km/sec. I believe that’s how fast our star goes around the merry go round.

Click to expand...

You moved the decimal point. Our speed around the merry go round is 230km/s.

But at that distance and time, that corresponds to over 500km/s!!!

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Fast, but not all that fast. Here's one doing 1200km/s:


And that's out in the middle of nowhere -- there are even faster ones in tight orbits around our central black hole.

steve_bank said:

Probably a dumb question.

What is the reference point for astronomical motion and velocity?

Click to expand...

For position, the barycenter of the Solar System. For direction, quasars, because they are the farthest objects that can be observed without much difficulty. The one with the highest apparent visual luminosity, 3C 273, has visual magnitude +12.9.

Others are not quite as bright in visible light. From  List of quasars and looking at the first-discovered ones, 3C 48: +16.2, 3C 47: +18.1, 3C 147 +17.8, 3C 9 +17.62.

I'll now consider how close a catalogued star can be to any one of them. I will use  Sirius as my reference.

•  Bayer designation - Alpha Canis Majoris - visibility: unaided eye
•  Flamsteed designation - 9 Canis Majoris - visibility: unaided eye, bright telescopic
•  Bright Star Catalogue (Harvard Revised) - HR 2491 - stars: 9,095, mag limit: +6.5 -- 0.22/dsq
•  Durchmusterung ("Survey") - BD -16d1591 - stars: 342,198 (Bonner D: 90dN-2dS) -- 17/dsq
•  Henry Draper Catalogue - HD 48915 - stars: 359,083, mag limit: +9 -- 8.7/dsq
•  Hipparcos - HIP 32349 - entries: 118,218, mag limit: +12.4 (complete) +7.3 - +9.0 -- 2.9/dsq
•  Tycho-2 Catalogue - (?) - entries: 2,539,913, mag limit: +11.5 (90%) +10.5, (99.9%) +10.0 -- 62/dsq
    dsq = square degrees in the sky Since quasars are not very bright in observation, one measures their positions relative to catalogued stars and uses those measurements to find those stars' positions relative to an inertial, nonrotating frame. Quasars are used because they are too small to for us to resolve at their distances, unlike galaxies.

Fixing that last one: dsq =  Square degree = square degrees in the sky Since quasars are not very bright in observation, one measures their positions relative to catalogued stars and uses those measurements to find those stars' positions relative to an inertial, nonrotating frame. Quasars are used because they are too small to for us to resolve at their distances, unlike galaxies.