# Is S2 still the fastest known star in the galaxy?

Wikipedia's entry for the star S2 says that it has

the fastest known ballistic orbit, reaching speeds exceeding 5,000 km/s (11,000,000 mph, or ​1⁄60 the speed of light) and acceleration of about 1.5 m/s2 (almost one-sixth of Earth's surface gravity).

But the referenced link to the press release Surfing a Black Hole is dated 2002. Have any stars with faster orbital speeds at perimelasma (periapsis for a black hole orbit) in the 19 years since this press release? Is it possible that there is a star on an even faster orbital trajectory than S2 or has any such possibility been ruled out?

• I've just asked Was GRAVITY built to look at one star? and hopefully you will ask when S62 will next pass periapsis and if it was measured by GRAVITY on it's previous and if not why not; I need to disconnect from SE for a few hours and get something done!
– uhoh
Apr 3 at 5:23
• You’re talking about speed relative to what? Apr 5 at 7:44
• I am asking about the speed relative to the black hole center of mass. We could use the orbital system center of mass instead, but that would be similar since the black hole is much more massive than the star. Apr 5 at 15:45

S0-2 (I assume you mean the star orbiting the Milky Way central black hole) actually reaches speeds of 7650 km/s at periapsis.

However, if you read further down the wikipedia article you will see that there are stars S0-102 and S62 which have shorter orbital periods than S0-2 (11.5 and 9.2 years vs 16.05 years).

Star S62 is also in a highly elliptical orbit ($$e=0.976$$, Peißker et al. 2020) and has a periapsis distance that is even closer to the black hole than S0-2, where it has a speed clocking in at about 30,000 km/s$$^{1}$$. No actual value or error bar is given in the paper.

The plot shows its orbit (in red) compared with other stars in the region.

STOP PRESS

A telegram issued by Peißker et al. 2020 claims detection of a new winner - S4714. They appear to have revised downwards their estimate of the peri-centre speed of S62 to 20,000 km/s and say that this new, very faint object, has a peri-centre speed of 24,000 km/s.

$$^1$$ Of course that is the speed as seen by a distant observer. If you were to go there and put a speed gun on it as it went past, then it would be a touch larger.

• From that plot alone, the fit for the $\color{red}{\text{red orbit}}$ looks pretty weird; it's certainly not the ellipse-of-best-fit, as it'd seem more sensible if the left-hand-side were slanted upward a bit. Guessing that the plot's informed by other factors that'd make the plotted orbit seem more sensible than it appears?
– Nat
Apr 3 at 11:03
• @Nat the position of Sgr A* is known and fixed in their fitting. Apr 3 at 11:28
• These ellipses do not look confocal to me. Especially if the red, green and orange ones may be nearly confocal, their common focus is not anywhere near the focus of the blue one, no? Apr 3 at 23:03
• Perhaps you are forgetting these are the projections of ellipses onto the plane of the sky? @მამუკაჯიბლაძე Apr 3 at 23:07
• Thanks! I did not realize that (even if they are all in the same plane) confocality is not preserved under projections Apr 4 at 4:43

There are a couple of stars with smaller orbits (and hence faster average speeds) S55 and S62. Of these, S62 has a faster speed at periapsis, approximately 10% of the speed of light:

the orbital time scale is measured to be 9.9 years, the next peri-center passage will be around March 2023. During that passage, the star will have a velocity of about 10% the speed of light. (S62 on a 9.9-year orbit around SgrA*. Florian Peiβker, Andreas Eckart and Marzieh Parsa, paper in preprint in Feb 2020)