Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_banner_terminator_challenge.thumb.jpg.b7f10f594317507d0f40662231b0d9a8.jpg

Ben478

Why does Sirius sparkle with different colours?

Recommended Posts

OK I know, beginner's question time now but I'm not afraid of asking! :D

I've observed Sirius on two occasions now and notice how it shines with a pure, brilliant silvery white colour, but also how it sparkles with other colours such as red and blue. Is this the star itself or as I suspect, the effect of the mirror and/or glass of the telescope or eyepiece? Many thanks, Ben. :)

Share this post


Link to post
Share on other sites

Sirius twinkles because it is so low down to the horizon as seen from the UK. The light from it has to travel through a lot more atsmosphere which is full of dust and dirt. This is what gives it its colours. On nights when you see it as white, i guess this is a good indication of good "seeing" conditions.

Edited by LukeSkywatcher
  • Like 1

Share this post


Link to post
Share on other sites

Although the dust and dirt does change the colour of things, making them typically appear redder, the cause of the twinkling and colour flashing is due to the light from the star passing through layers and blobs of air with different light bending properties. As the light passes through this air it can be shifted by a small amount 'off-axis' if you like, and this causes a momentary change in the amount of light reaching your eye. The star appears to twinkle.

The bending can also be accompanied by the light being spread out into its component spectral colours and as this spectrum can also shift to the side, your eye picks the effect up as a flash of colour.

In theory the lack of twinkling (technical name for it is scintillation) could be seen as a sign of good seeing and I've had great views of the Moon and planets after using just that assumption. However, I've also had rubbish views when the stars have appears very steady in the sky so it's not a hard and fast rule at all.

Dust and dirt in the atmosphere isn't directly related to seeing but rather the clarity of the atmosphere, as described by the sky's "transparency".

Share this post


Link to post
Share on other sites

I should also point out that the reason why Sirius appears to flash so much is that it's very bright and low. Being low, the amount of atmosphere it has to travel through is much thicker and hence there's more opportunity for the light from the star to get interfered with.

Share this post


Link to post
Share on other sites

Thank you both for your replies, it makes perfect sense now! So even though it's atmospherics to blame I think it makes Sirius even more beautiful to observe. It's just like a diamond when it sparkles! :)

Edited by Ben478
  • Like 1

Share this post


Link to post
Share on other sites
No problem - an easy thing to do.

You are too kind. I fear it is just me though. I have a terrible habit also of confusing Nagler EPs with Kelner. I usually end up saying something like "The nagler EPs that come supplied with most scopes are pretty bad".

Share this post


Link to post
Share on other sites

Splendid answer from Pete. It would be interesting to see Sirius from much further south so as to see iit near the zenith. An non-scinillating Sirius would be most unfamiliar.

Olly

  • Like 1

Share this post


Link to post
Share on other sites

Our eyes can't detect colour if the source is dimmer than some threshold. We don't see the scattered colours from other stars because they're not as bright as Sirius even when they are near the horizon.

  • Like 2

Share this post


Link to post
Share on other sites

For a real challenge you could have a go at seeing the extremely

difficult white dwarf companion that orbits Sirius.

Here's a drawing I made last year, the Pup was visible in the glare of the Sirius.

post-17619-133877422241_thumb.jpg

  • Like 3

Share this post


Link to post
Share on other sites

It's beautiful when they flash like that isn't it, Ben? I've also seen it happen with Capella, Rigel, Vega, Spica and Venus. At least our atmospheric gunk is worth something. :)

Share this post


Link to post
Share on other sites

Hi! Im viewing sirius from Vancouver Island, Canada. 49th parallel. It is flashing colours like crazy! It is definitely not low in the sky right now. Could it be because it's much colder than usual tonight? Maybe it really does flash different colours? So beautiful.

Share this post


Link to post
Share on other sites

It's called scintillation and has nothing to do with Sirius itself, which is a stable star. The atmosphere is turbulent and causes diffraction which will affect the different wavelengths of light differently, causing the beam from Sirius to move around, dim and brighten, and change colour. The lower a star is in the sky the more atmosphere its beam has to pass through, so scintillation decreases as we look upwards towards the zenith. The stability/instability of the atmosphere at a particular time is known as the 'seeing' - which is highly variable. When it's bad there is no point in using high magnification. 

Planets don't twinkle because they are discs of light rather than points, like stars, so they send out many parallel beams each of which are affected randomly by the atmosphere so that the scintillation averages itself out.

Sirius itself does not change colour or brightness rapidly. This is quite complicated but there are no physical processes in stars which can bring about changes on that timescale. 

Olly

Edited by ollypenrice
  • Like 1

Share this post


Link to post
Share on other sites
On 25/01/2010 at 21:25, Towa said:

For a real challenge you could have a go at seeing the extremely

difficult white dwarf companion that orbits Sirius

Kudos indeed! I tried this for some time a few weeks back but no joy. Also tried mucking about with camera settings to see if I could image it instead but again didn't manage it. One of those fails where you don't really mind 'coz you had fun trying.

  • Like 1

Share this post


Link to post
Share on other sites

Wanted to know this as well thanks for bringing it up can always see it with red/blue sparkle with the naked eye but never knew why. :) 

Edited by Dinoco

Share this post


Link to post
Share on other sites

Yes Sirius B is a very challenging target in the UK with Sirius so low in the sky! I finally managed it visually a couple of nights ago with my 7" refractor but I've tried many times before to no avail. It all comes down to seeing. In moments of very steady air it's right there and relatively obvious but most of the time it's lost in a sea of scattered and dispersed light from Sirius. For some reason the atmospheric dispersion and turbulence tend to smear the light into a flame or fan shape like a peacock tail, which would be very pretty if it's wasn't masking what you were looking at!

5a903efdc73c4_SketchesofSiriusBina722refractor.jpg.fde29e05d2e97f8bc32a9e2d0ea6d12b.jpg

  • Like 1

Share this post


Link to post
Share on other sites

Down here in the southern Alentejo, Portugal at more or less 37 degrees compared to southern UK 51 degrees North, Sirius is way up high right now, and sparkling like crazy anyway. I love the interaction with the atmosphere, with or without a telescope, but most of all through a telescope way off focus, like a plasma ball, so beautiful!

Share this post


Link to post
Share on other sites
On 19/02/2018 at 13:21, spaceman_spiff said:

There's a great article on the subject:

http://www.skyandtelescope.com/astronomy-blogs/sirius-ly-scintillating-holiday12222014/

@Eriska- here's a photo from the article showing the apparent colours of Sirius.

image.png.283221528be645e73e76ed5471bfbd0c.png

That is a cool set of images, shows how much scintillation can change the colour of the light!

As an aside, that middle image looks a lot like Mars! And now since I've thought that, all the others now look like the old versions of the planets you saw on the original series of Star Trek before they updated the effects!:happy7::headbang:

Edited by trynda1701
  • Like 1

Share this post


Link to post
Share on other sites

Sirius also appears to sparkle much more because it is so bright and is a point source of light.  Of course, stars with a similar declination will also twinkle but less so than Sirius.

The planets do not twinkle nearly as much (though do a bit) because they are disks.

Share this post


Link to post
Share on other sites

Hey love that you posted this as you are not far from me,  (cowichan valley) and I see and enjoy serius sparkling all the time.  I've often wondered why it did.  Thank you all for sharing your knowledge! 🙂

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.

×

Important Information

By using this site, you agree to our Terms of Use.