Hubble’s Ultra Deep Field Activity
How do we get this image?
a.The HUDF image was made by looking at the same area in space for 11 days. Why so long?
Many of the objects are very far away and, consequently, very faint. The longer you open yourcamera, the more light it collects, and the fainter the objects you can see. Oftentimes, studentsget confused between light-travel time and length of exposure, even though these are on totally
different time scales. Key point here: we’re not waiting to receive light from more distantsources, we are maximizing the amount of light from all sources, so that we can detect very faint(distant) ones.
b.The HUDF was intended to reveal objects in the distant Universe. Where on the sky would they have pointed the telescope?
Away from bright nearby sources! These would include nearby stars in our galaxy and nearby galaxies. The region of sky that surrounds the HUDF is shown on one of the slides provided.
c.The HUDF image was taken in visible light.
What types of objects are there in the image?
What type or types of object predominate? How would you describe the “typical” object in this image?
Students should realize (or you may have to lead them to realize) that nearly all objects in the image are galaxies.
Objects that are not galaxies in this image are stars - these are easily picked out by their crosslike spikes in the image. These spikes are known as diffraction spikes and are caused by the struts supporting of the secondary mirror in the telescope, and only arise when imaging a bright,
concentrated source. Galaxies are usually not bright and concentrated enough to cause this effect. The stars in this image are foreground stars in our own galaxy.
How many objects are there in the image?
- Come up with a practical procedure for estimation and then carry it out. Do it several times independently (i.e., each person in the group) to get an estimate of how results may vary.
Students may come up with a range of answers on this question, but you should guide them (as they work in their group) toward splitting the image into squares, counting the number of galaxies in one square, and then multiplying this number by the number of squares. The most likely result will be a slight undercount, coming in around 5,000, while the true number is approximately 10,000.
- Think about errors that may occur in your method, and suggest one source of random error and one source of systematic error.
One source of random-like error will be the particular portions of the image that students sample. Because of the uneven distribution of objects, there will be variation in the estimates obtained from different portions. One source of systematic error would be judgment about what qualifies as an “object” – i.e., if we are picky about what we include, we will miss really faint objects. On the other hand, if we are too liberal, then we may count “objects” that are just noise.
- Assuming there are 100 billion galaxies in observable universe, approximately what fraction of the sky must the HUDF cover, given your estimate for the number of objects?
The estimate for the number of galaxies in the iamge should come out in the ballpark of 5,000-
10,000. The “official Astronomer answer” is just over 10,000. Thus 10,000/100x10^9 = 10^-7
in other words, 1/10 million – that’s the fraction of the sky covered!
Thought Experiment:
How many planets are there in the observable Universe?
If we tell students that an average galaxy has ~1011stars.
In the end you should get a variety of answers to the following question, but hopefully a fairly narrow range of at most 2 or so orders of magnitude. If they use their own estimates all the way through the calculations, the range may be larger.
Depending on what is assumed for the frequency of occurrence of planets around stars, the estimate should come out on the order of:
(1011 gal/Univ) * (1011stars/gal) * (0.5 stars w/ planets) * (8 planets/star) = 4 x 1022planets!
Assumptions:
(1) The image was representative - a fairly fundamental assumption in cosmology (so much so that it's known as the "cosmological principle" -- that the Universe is homogeneous andisotropic) -- in a modern understanding this is only true when averaging over fairly large regions of the Universe.
(2) All galaxies in this region of sky are detected in the image/all sources in the image aregalaxies.
(3) Only 50% of stars in a galaxy host planets
(4) All of these stellar systems are like our solar system – i.e. 8 planets