In a controlled experiment, an independent variable (the cause) is systematically manipulated and the dependent variable (the effect) is measured; any extraneous variables are controlled. The researcher can operationalize (i.e. define) the variables being studied so they can be objectivity measured.
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At the center of the cloud, more weight of the gas molecules are present.
The mass or weight at the center of the cloud increases when the gas molecules group together in a molecular cloud because more gas molecules are present at the center which combines with each other leads to heavier weight of gas molecules. The combination of gas molecules make the center of the molecular cloud denser as compared to other parts of the cloud so we can conclude that increase in mass occur at the center of the cloud.
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Let us look at the what, why and how of studying extreme places like the deep earth and the deep space. Scientists need to figure out as to 'what' they want to look at such places, be it a new exotic creature or bio-genesis (birth of life). Unless there are several testable hypothesis constructed, such a study cannot begin. The 'why' aspect deals with the purpose of such research and expeditions. Is it of any use to the humans, or will it improve our current understanding of a phenomenon? The 'how' aspect deals with the technology and the economic assistance that can help in undertaking such a research. All these are the challenges that needed to be thoroughly considered to make such a research or expedition possible.