question , starting with how to quantify the trend and then determine the procedure themselves for making the calculations below . Using a calculator , find the percentages of each galaxy type in the cluster versus the field ( Ignore IRs and INTs ).
Use your numbers from the table above to calculate the percentages and fill in each of these blanks below : In the Cluster :% of Ellipticals ( e / h ) = % % of Lenticulars ( f / h ) = % % of Spirals ( g / h ) = % In the Field :% of Ellipticals ( i / m ) = % % of Lenticulars ( j / m ) = _ % % of Spirals ( k / m ) = %
Question : Where did you find a higher percentage of spirals - in the Cluster or in the Field ? Answer : _ Tell students : The percentages that you just found tell us which types of galaxies are common in the Coma Cluster versus which types are common in the field .
Astronomers have done this same experiment on hundreds of thousands of galaxies in the nearby universe , and discovered that the following percentages are pretty typical :
ˆ In dense clusters , 40 % of the galaxies are ellipticals , 50 % are lenticulars , and 10 % are
spirals . ˆ In the field , 10 % of the galaxies are ellipticals , 10 % are lenticulars , and 80 % are spirals .
When galaxies are found very close together there are more ellipticals and lenticulars . When galaxies are far apart there are more spirals . Astronomers call this the " morphology-density effect ." This term basically means that in crowded galaxy neighbourhoods , like clusters , there are different types of galaxies than are found in open areas , like the field .
Step 9
Students should by now ( from Step 7 ) have asked the question , " Why do we see more elliptical and lenticular galaxies in clusters and more spirals in the field ?" ( This question can also be phrased , " Why do we observe the morphology-density effect ?") They should also have had the idea that interactions could be involved , and maybe even the idea that more interactions take place in denser environments , like the center of a cluster . Below is information that can be used to answer this question . You can give students this text to read , then ask them to discuss and write down an explanation for this effect ; or you can continue to prompt students to brainstorm and discuss ideas for possible explanations , then potentially have them do research in textbooks / on the internet on their own or in groups , and then have them share their explanations with each other .
Explanation : Many galaxies contain what astronomers call " gas ," which generally means hydrogen gas , sometimes mixed with the gases of other elements , and sometimes mixed also with dust . Gas clouds can collapse by gravity , which leads to the formation of stars . Astronomers have observed many spiral galaxies ( S and SB ) and find that most of these galaxies contain a lot of gas , and are currently forming lots of new stars . Elliptical and lenticular galaxies ( E , S0 , and SB0 ) are gas-poor and are not making many new stars . Galaxies that are very close to each other , such as those in clusters , often undergo many violent interactions with each other . When a gas-rich spiral galaxy interacts with another galaxy , it tends to quickly use up most of its gas to make new stars , leaving little gas behind . Galaxy-galaxy interactions often change gas-rich galaxies into gas-poor galaxies . Many lenticular galaxies are the remains of old spirals that have lost their gas , and many elliptical galaxies are the remains of several spiral galaxies that have collided . Galaxy clusters are usually filled with a lot of extremely hot gas that is spread between galaxies throughout the cluster . However , there is no hot gas like this out in the field . When the radiation from this hot gas hits a spiral galaxy , it strips the spiral galaxy of its much cooler gas
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