Please read the following table in HTML format from a webpage and then answer the question in detail according to the table. Each table cell is defined by a <td> and a </td> tag. Each table row starts with a <tr> and ends with a </tr> tag. 'rowspan=m' and 'colspan=n' represents a cell spaning over 'm' rows and 'n' columns.

Webpage title:
Radioisotope thermoelectric generator
Table title:
Criteria for selection of isotopes
Table:
<table id='hittask' contenteditable='False'>
<tr> <th colspan=1 rowspan=1 > Material </th><th colspan=1 rowspan=1 > Shielding </th><th colspan=2 rowspan=1 > Power density (W/g) </th><th colspan=2 rowspan=1 > Half-life (years) </th></tr>
<tr> <td class="highlighted"  colspan=1 rowspan=1 > 238Pu </td><td colspan=1 rowspan=1 > Low </td><td colspan=1 rowspan=1 > 0.54 </td><td colspan=1 rowspan=1 >  </td><td class="highlighted"  colspan=1 rowspan=1 > 87.7 </td><td colspan=1 rowspan=1 >  </td></tr>
<tr> <td class="highlighted"  colspan=1 rowspan=1 > 90Sr </td><td colspan=1 rowspan=1 > High </td><td class="highlighted"  colspan=1 rowspan=1 > 0.46 </td><td colspan=1 rowspan=1 >  </td><td class="highlighted"  colspan=1 rowspan=1 > 28.8 </td><td colspan=1 rowspan=1 >  </td></tr>
<tr> <td colspan=1 rowspan=1 > 210Po </td><td colspan=1 rowspan=1 > Low </td><td colspan=1 rowspan=1 > 140 </td><td colspan=1 rowspan=1 >  </td><td colspan=1 rowspan=1 > 0.378 </td><td colspan=1 rowspan=1 >  </td></tr>
<tr> <td colspan=1 rowspan=1 > 241Am </td><td colspan=1 rowspan=1 > Medium </td><td colspan=1 rowspan=1 > 0.114 </td><td colspan=1 rowspan=1 >  </td><td colspan=1 rowspan=1 > 432 </td><td colspan=1 rowspan=1 >  </td></tr>
</table>
Question:
What is 90Sr's half life?
Answer:
90Sr's half life of 28.8 years is shorter than that of ²³⁸Pu and it also has a lower a power density of 0.46 watts per gram.