Lost in Space

Lost in Space

ASTR1001 Assignment 5

Introduction

The Starship USS Drongo was rescued from planet Ziggy by a benevolent race of aliens.

Unfortunately, just after the rescue, the aliens happened to notice the music video collection of one of the junior engineering officers.

Enraged that they had rescued life-forms with such disgusting musical taste, the aliens immediately flung the USS Drongo through a wormhole.

USS Drongo in the Wormhole

Lost in Space!

The USS Drongo is now floating somewhere in space. You do not know if you are in some remote part of our own

universe, or some quite different universe. Captain Howard has called you, the astrophysics team,

together. Your task: work out whether we are in our own universe, or

whether we are in a different one. Investigate our surroundings. If we are in our own universe,

what sort of environment could we be in? If we are in a different universe, try and work out its cosmology,

as this will help the computer calculate a way home.

The View

The view around the spacecraft looks quite empty: there do not appear to be any nearby planets, asteroids, suns etc.

The space surrounding your ship is a hard vacuum. With the naked eye, you can see several thousand

stars: about the same number that you could see from Earth.

Funny Colours The sky is not, however, quite like that seen from Earth. The stars seem to be rather more colourful than those seen

from Earth. In particular, they seem to show bright primary colours, rather than the more subtle shades seen from Earth.

Red stars seem to outnumber blue ones by quite a bit. There is no sign of a Milky Way. Curiously, the stars appear to twinkle, which they should not

do in the absence of an atmosphere. Just like from Earth, however, the stars appear as dots, even

through the best on-board telescopes: no disks are seen

Radio Signals

Curiously, the signals division are reporting many sudden, sharp bursts of radio emission.

These bursts mostly occur at wavelengths of 20cm or slightly longer.

They are extremely intense while they occur, but last for only microseconds.

Many bursts are very intense. They occur several times a day.

There also appear to be a much larger number of faint bursts, at somewhat longer wavelengths.

Spectroscopy

The instrumentation division have just jury-rigged a spectrograph, and obtained spectra of some stars.

The spectra are VERY unusual. All emission seems to occur at one particular

wavelength.

Wavelength

Flux

Wavelengths Each different star appears to radiate at a different wavelength. The wavelengths can lie anywhere in the whole wavelength

range that the detector is sensitive to: 400 - 800nm, though they do show a bias towards redder wavelengths.

As far as we can tell, the flux from a given star could be purely monochromatic: all the radiation comes at precisely one wavelength. The line width in the previous graph is due to the limited instrumental resolution of the spectrograph. We place an upper limit on the line width of 1nm.

No other emission lines are seen from any stars: down to a flux limit of 0.1% the intensity of the main line.

Twinkling

Why do the stars twinkle, in the absence of any atmosphere?

You rigged up a multichannel photoelectric photometer to measure the brightness of some stars as a function of time.

Time

Flux

Rapid Pulsation

It appears that all the stars around you are pulsing rapidly. Pulse periods are typically a few seconds, and pulse amplitudes a few percent or less.

Not all stars have the same periods and the same amplitudes.

You have measured the pulse periods and amplitudes (maximum flux minus minimum flux divided by the average flux). The results are found in a separate Excel data table.

The Parallax Probe

To aid your investigation, Captain Howard sent out a robotic space-probe.

The probe travelled 10,000 km away, and then stopped and deployed a small telescope.

It measured the positions of dozens of bright stars with an accuracy of 0.2 arcseconds.

At the same moment, you performed the same measurement using the USS Drongo’s main telescope.

The Geometry10

,000

km You studied

stars in this direction

The Data Table

Between your two measurements you were able to measure whether the stars appeared to lie in exactly the same direction from the USS Drongo and from the probe.

You found that many of the brightest stars appeared in slightly different directions.

So the light from these stars was not coming in quite parallel to you and to the probe: you measured the difference in angle (the parallax) with an accuracy of about 0.2 arcsec.

Data from these observations can also be found in the separate Excel spreadsheet.

Your Mission.

Captain Howard has called you, the astrophysics team, together.

Your task: work out whether we are in our own universe, or whether we are in a different one.

Investigate our surroundings. If we are in our universe, where could we be? What is our environment like?

If we are in a different universe, try and work out its cosmology, as this will help the computer calculate a way home.

Assessment

This assignment is due by 10am on Thursday 29th May. It is worth 10% of the marks for ASTR1001. You should describe what you have learned about the

universe in which you now find yourselves. Word limit: 2000 words. You should describe the most important things you

have learned, and describe how you learned them. There is no need to show mathematical working.

You should also briefly describe what future observations would be most useful.

More assessment details

As usual, you can work by yourselves or in groups. You can submit your assigments electronically via

WebCT e-mail, or in person to me in class, or into my mailbox in the physics department.

As usual, please use the bulletin board to exchange ideas. I may hand out a few bonus marks to those who make particularly good postings.

Assignment 6

Once your assignments are in, I will release more data on the USS Drongo and its surrounds.

This further data will be the subject of Assignment 6.

I will also release a model answer to Assignment 5, to make sure that you are in a good position to attempt Assignment 6, even if you stuffed this one up.

Marking

To get maximum marks, you should deduce as much as possible from the data, but not deduce more from the data than is possible!

I’m after hard numbers and not just waffly text (this is a science course, after all…)

Feel free to speculate, but make it clear what is speculation and what is solid deduction.

I’ve put some numerical data in a spreadsheet to facilitate playing with it. If you’d like it in some other format, let me know (eg. text files for plotting/processing using MATLAB). You may or may not find this data useful.

The Data

The data file contains information on a bunch of bright stars that you have observed.

For each star it lists: A catalogue number The parallax (angular difference between the position

measured by you and by the probe) Wavelength at which it is emitting. How strong the average emission you detect from that star

is. How rapidly it pulses. By how much its intensity changes during a pulsation.