Name: ______Date: ______

Student Activity 5—Sheet 1

Crab Nebula Expansion

Science Background

Your teacher has given you two images of the Crab Nebula. The first was taken on February 11, 1956, and the second was taken on November 10, 1999. The images are negative images—bright objects appear dark and dark objects appear bright. The scale of the pictures is the same, so you should notice that the image from 1999 appears larger than the image from 1956. This is due to the expansion of the nebula that has taken place over the years. Notice that 10 individual knots in the cloud of gas and the pulsar that lies at the centre of the explosion are marked.

Measure

1.On the 1956 image, draw a line from the centre of the pulsar to the centre of knot 1. (Knot 1 is labelled with a pair of vertical dashes and is at the centre of those dashes.) Measure the distance from the pulsar to knot 1. Record this measurement in column 2 of Table 1. Important:Record your answer in centimetres, accurate to the nearest half millimetre. For example, record a reading between 3.6 cm and 3.7 cm as 3.65 cm.

2.Repeat Step 1 for knot 1 in the 1999 image. Record this measurement in column 3 of Table 1.

3.Repeat Steps 1 and 2 for knots 2 through 10, each time measuring the pulsar-to-knot distance on the 1956 image and then the corresponding pulsar-to-knot distance on the 1999 image.

Calculate

4.Calculate how far each knot has moved in the images from 1956 to 1999, and record your result in column 4 of Table 1.

Change in position  Distance from pulsar in 1999 – Distance from pulsar in 1956

5.The first image was taken on February 11, 1956, and the second was taken on November 10, 1999. Calculate the number of years between the two images, accurate to two decimal places.

Time between photos _____ years ____ months
 ______years

Next, for each knot,calculate the rate at which the nebula is expanding (in centimetres per year), accurate to five decimal places. Record your result in column 5.

6.Now that we know how fast each knot is moving in the images, calculate how many years it took each knot to travel from the pulsar to its place in the 1999 image. Round your answer to the nearest year. Record your result in column 6.

7.Based on your calculations, each knotwill give an estimated year for the supernova explosion that created the Crab Nebula. Determine this estimate for each knot and record it in column 7, accurate to the nearest year.

Estimated year of supernova 1999 – Elapsed time since explosion

8.To come up with a best estimate for the year of the explosion, find the average of the years in column 7.

Table 1

Column / 2 / 3 / 4 / 5 / 6 / 7
Knot # / Distance from Pulsar in 1956 (cm) / Distance from Pulsar in 1999 (cm) / Change in Position of Knot
(cm) / Rate of Expansion of Knot
(cm/year) / Elapsed Time from Explosion to 1999 (years) / Estimated Year of Supernova
1
2
3
4
5
6
7
8
9
10
Average

Summarize Your Learning

It is generally believed that the supernova was seen on Earth in 1054 C.E. It was so bright that it was visible during the day for 23 days afterwards and with the naked eye at night for almost two years afterwards.

1.What reasons could there be for the difference between the date you estimated and the actual date of the supernova sighting?

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2.How could we do a better job of estimating?

______

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3.The Crab Nebula is so far away that the light from the supernova took about 6500 years to reach Earth. Approximately what year on Earth did the supernova really take place?

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______

Post-Activity Assessment

Answer the following questions to check your understanding of concepts and skills related to calculating the age of the Crab Nebula, including measurement, ratio and proportion, and elapsed time.

1.Two images of a distant nebula show that a knot of gas has moved by 2.4 cm in 6.4 years. On the images, 20 cm represents a distance of 150 million (150000000) km. How fast was the knot actually moving in

(a)kilometres per year? ______

(b)kilometres per hour? ______

2.Moreton waves are types of shock waves seen on the Sun. (They look like rings in the images below.) They are caused by violent solar events, such as solar flares, and travel at high speeds and across vast distances. A solar flare on July 9, 1996, caused a Moreton wave to travel across the Sun’s surface, as shown below. Each picture is equivalent to 200 million metres on a side. The difference in time between the images is 1 hour.

Credit: ESA/NASA

What was the speed of the wave in

(a)metres per hour?______

(b)kilometres per hour?______

(c)kilometres per second?______

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