The maximum temperature the CPUs can reach before causing damage

The max case temperature is 72.4C. The processor itself will likely be hotter than that. (source: http://processorfinder.intel.com/details.aspx?sSpec=SLB9L#).

If your software is reporting a core temp of 50C you should be fine.

If you have added additional hard drives, a beefier video card or more power thirsty RAM, it may be that your power supply is too small (too low wattage). Other potential causes are bad sticks of RAM.

As others have noted, you need the lm-sensors package. I thought I should mention that in addition to installing the package, you should run the sensors-detect command to identify the kernel modules that need to be loaded.

If you haven't done this, manually run this once and check if it detects any sensors on board your computer.

$ sudo sensors-detect 
# sensors-detect revision 5249 (2008-05-11 22:56:25 +0200)

This program will help you determine which kernel modules you need
to load to use lm_sensors most effectively. It is generally safe
and recommended to accept the default answers to all questions,
unless you know what you're doing.

We can start with probing for (PCI) I2C or SMBus adapters.
Do you want to probe now? (YES/no): 
Probing for PCI bus adapters...
Use driver `i2c-i801' for device 0000:00:1f.3: Intel 82801G ICH7

We will now try to load each adapter module in turn.
Module `i2c-i801' already loaded.
If you have undetectable or unsupported I2C/SMBus adapters, you can have
them scanned by manually loading the modules before running this script.

We are now going to do the I2C/SMBus adapter probings. Some chips may
be double detected; we choose the one with the highest confidence
value in that case.
If you found that the adapter hung after probing a certain address,
you can specify that address to remain unprobed.

Next adapter: SMBus I801 adapter at 10c0 (i2c-0)
Do you want to scan it? (YES/no/selectively): 
Client found at address 0x50
Probing for `Analog Devices ADM1033'...                     No
Probing for `Analog Devices ADM1034'...                     No
Probing for `SPD EEPROM'...                                 Yes
    (confidence 8, not a hardware monitoring chip)
Probing for `EDID EEPROM'...                                No
Client found at address 0x52
Probing for `Analog Devices ADM1033'...                     No
Probing for `Analog Devices ADM1034'...                     No
Probing for `SPD EEPROM'...                                 Yes
    (confidence 8, not a hardware monitoring chip)
Probing for `EDID EEPROM'...                                No

Some chips are also accessible through the ISA I/O ports. We have to
write to arbitrary I/O ports to probe them. This is usually safe though.
Yes, you do have ISA I/O ports even if you do not have any ISA slots!
Do you want to scan the ISA I/O ports? (YES/no): 
Probing for `National Semiconductor LM78' at 0x290...       No
Probing for `National Semiconductor LM78-J' at 0x290...     No
Probing for `National Semiconductor LM79' at 0x290...       No
Probing for `Winbond W83781D' at 0x290...                   No
Probing for `Winbond W83782D' at 0x290...                   No
Probing for `IPMI BMC KCS' at 0xca0...                      No
Probing for `IPMI BMC SMIC' at 0xca8...                     No

Some Super I/O chips may also contain sensors. We have to write to
standard I/O ports to probe them. This is usually safe.
Do you want to scan for Super I/O sensors? (YES/no): 
Probing for Super-I/O at 0x2e/0x2f
Trying family `National Semiconductor'...                   No
Trying family `SMSC'...                                     Yes
Found unknown chip with ID 0x2803
Probing for Super-I/O at 0x4e/0x4f
Trying family `National Semiconductor'...                   No
Trying family `SMSC'...                                     No
Trying family `VIA/Winbond/Fintek'...                       No
Trying family `ITE'...                                      No

Some south bridges, CPUs or memory controllers may also contain
embedded sensors. Do you want to scan for them? (YES/no): 
Silicon Integrated Systems SIS5595...                       No
VIA VT82C686 Integrated Sensors...                          No
VIA VT8231 Integrated Sensors...                            No
AMD K8 thermal sensors...                                   No
AMD K10 thermal sensors...                                  No
Intel Core family thermal sensor...                         Success!
    (driver `coretemp')
Intel AMB FB-DIMM thermal sensor...                         No

Now follows a summary of the probes I have just done.
Just press ENTER to continue: 

Driver `coretemp' (should be inserted):
  Detects correctly:
  * Chip `Intel Core family thermal sensor' (confidence: 9)

I will now generate the commands needed to load the required modules.
Just press ENTER to continue: 

To load everything that is needed, add this to /etc/modules:

#----cut here----
# Chip drivers
coretemp
#----cut here----

Do you want to add these lines automatically? (yes/NO)

If you're lucky, your laptop will support an array of sensors. If not, you may still find a basic set of sensors for the CPU:

$ sensors
acpitz-virtual-0
Adapter: Virtual device
temp1:       +40.5°C  (crit = +105.0°C)                  

coretemp-isa-0000
Adapter: ISA adapter
Core 0:      +35.0°C  (crit = +100.0°C)                  

coretemp-isa-0001
Adapter: ISA adapter
Core 1:      +40.0°C  (crit = +100.0°C)